Summary: a whole-hearted career is usually not achieved in part-time. Neither in academia nor in industry.
The exception I can see, though, is if the start-up is closely related to the founder's field of research. In that case, the professional experience from both academia and business may sucessfully add up, and can be transferred from one to the other, e.g. academic research going into high-tech business, and business experience providing knowledge such as "this works in practice and also over extended time".
(I'd definitively not take the full plunge in your situation - unless a founder grant comes up that pays you for developing the business full time. But that is something that you'll have to decide for yourself.)
Disclaimer: I'm business owner, still quite close to academia (see below) and doing research, but not a start-up founder (in the sense that I'm not planning fast/explosive growth for the company)
IMHO there are some business models and circumstances that go well alongside academia/research and give synergy effects while other combinations don't combine as well/just exist in parallel without synergy.
E.g. my business now is pretty much the same as what I did as researcher: chemometrics, i.e. data analysis specialized to chemistry.
In contrast, your business idea is related to academia (you spotted a need there), but that experience is unlikely to help your civil engineering academic carreer (nor vice versa).
This will probably make the transition/distinction between academia and business harder/sharper for you. However, there's still quite a lot of room for you to decide how much of a sharp transition is is to be.
[Working part-time vs.] I keep hearing the advice that it's better to go all in and remove any safety net
That's of course up to you to decide. And as a startup founder, you may as well get used to making decision of that "size".
The most obvious advantage of going all in is developing the business faster. This may mean more profit/success earlier, and it may tip the scales to becoming successful at all.
I went the gradual way, very much so: I first had my business as a side-line that served also as having an immediate plan B to counter academic job insecurity, at the same time putting together quite some savings (by working while sticking to a student lifestyle - this easy when working in academia as large parts of the colleagues around are unpaid or on part-time wage), both with the primary goal of creating a level playing field for academic job negotiations. After some 10 years of experience in my field I finally decided it's time to move more seriously into business, and took the offer of my employer to "phase out" with a part-time contract looking a bit longer after one research project. Since then, I've had more part-time contracts at other academic institutes, which I mentally treat as clients, though: they had grants that called for employees but did not allow sub-contractors.
In that situation, the part-time academia/part-time self-employed has some burocratic disadvantages: I need non-standard working contracts that explicitly put formal boundaries around what is supposed to be the results of the employed work (otherwise e.g. copyright for software I write for a customer or for my company may end up being with the [academic] employer) - and university HR departments
may will probably block anything they are not used to ("We never do anything but standard contracts").
Back to the recommendation of plunging in: I'd also recommend to keep in mind how much skin those who advise you have in the consequences of that decision (i.e. Is it your parents who advise and would maybe take you into their home again if the business fails and you're deep in debt? Or who even say that they'd bail you out up to xxx SEK? Is it some friends for whom your financial desaster won't have the slightest consequence? Is it someone who could easily afford to loose, say, 1/4 mio SEK or 25 k€ or even more on the start-up experiment telling you what they'd do in your place?)
Of course if you go wholly in at a stage in life where you have hardly any savings, there isn't as much that is lost if that business fails. But you may be ending in a personal bankrupcy. Also, some business models have clients that take a deep look into the financial position of the owner, or that may require substantial financial stamina - my experience with universities as late paying customers suggests that the latter may be the case with your business as well.
There are also cultural aspects here: I'm in Germany where society is in general comparably risk-averse and failing as business owner (and much less personal bancruptcy) is not really the done thing.
Plus, what are the consequences in terms of social insurance of (not) having a side job? Here, the social security system in large parts revolves around the so-called normal employee. One of the peculiarities is that for self-employed the governmental health insurance has a minimum contribution (which was recently lowered so that the proportional zone starts now already at ≈1040 €/month), another one is that for self-employed all sources of income count while for employees only the wage is subject to health insurance fees.
In consequence, the recommendation here for most is to start a business from a part-time position if feasible (unless one is really sure that it will generate good income from the 1st year on, or one is paid by a business founder grant etc.).
All this aside, some startup statistics:
- the vast majority (> 3/4) of start-ups have their founders actually put savings into the business (EU Startup Monitor 2018, numbers in the US are AFAIK similar).
- It is normal for a startup to take years becoming successful. This doesn't mean the business won't be able to pay, but if you have the financial freedom to decide whether it's good to take out money to pay yourself vs. leaving that money in the business as investment you're obviously in a much better position than if you have to take out the money for your own survival.
And having a small but steady and sufficient income from some other source may give you that freedom.
Part time positions for business founders are again something that is often much easier available in academia than in industry, particularly if the reason for asking for a part-time job is that one wants to found a business in that profession: to academia that is usually not a competitor but a source of pride "that's our former colleague who's now a founder" or, if it's a spin-off, also something that counts in evaluations of the department.
Your situation of founding something totally different from your profession may help you to get an industry part-time job (paid with an industry wage) in parallel to your own business. So if you decide that keeping a foot inside academia isn't for your love of academic research but for practical financial reasons, you may consider a part time job in industry as alternative.
Last but not least, you'll have to specify what a successful career in academia is for you.
Personally, I like doing research - so for me a lifetime position as a researcher (as opposed to research manager such as professors usually are) would be a successful academic career. You may call that the eternal postdoc.
Even though I certainly do a lot of burocracy as business owner, I most probably would have spent similar amounts of time with grant writing and (other) burocracy had I stayed in academia as "advancing" from postdoc to PI would have been basically unavoidable.
Oh, and I teach (mostly industry clients but also at universities and research institutes) and I still do reseach and method development - in some ways I now have more freedom in that than the academic freedom I experienced in some research institutes. So I still have the parts of academic work that I like most.
What I think is not going to work is at the same time trying to get a startup successfully going and going for a highly competitive academic career (professorship/starting your own academic group - see also What ratio of PhD graduates in STEM fields ultimately end up as (tenured) professors?).
Unrelated business field and academia
This is only hear-say, so please take it as such. I've been told anecdotally by a chemistry professor close to retirement that it is not unusual for people being somewhat burned out/fed up with their profession after finishing their PhD and in consequence running an unrelated business (think pet shop by PhD in chemistry) for a while - until they return to their original profession. I don't recall whether anything was said about returning to academic research vs. working in industry (but a chemist with PhD in industry is likely to do research as well)