Not sure this answers the question, but at least I think I can provide some food for thought:
Your description of the situation sounds to me as if it may be substantially worsened by Covid regulation first asking and in many places still suggesting far more work from home than previously.
(I may say that this disconnect from colleagues has also affected my performance during the pandemic (whoopee, we have conferences again, in person) - but compared to you I do have the advantage that my PhD is long over and I am sure for myself that it is not a general crisis towards my work.)
(Also, I've made some experience in several groups from which I learned what amount of interaction with colleagues is good for me: having a more-or-less single place in a lab, but regular coffee break with the colleagues if I liked to; when I was "promoted" to my own 1-person-office abroad, that wasn't enough connection with the people there - they also had no space for coffee together - I moved into a 2 person office, and that was very good. 3 people with back to the open door - no good. 7 person office, no good at all, I went for lots of WFH... I've worked a lot online already before Covid, but I learned that I do need some amount of offline interaction as well, and I've sorely missed the random professional discussions that happen e.g. at conferences in person, but are extremely rare online)
The additional separation due to Covid may add up with the "disconnect" you describe by having a different background and the general stess of a thesis. While all three together may very well be too much, so something needs to be done, I wouldn't be sure from your description alone that quitting is the right step for you. This is something that should be considered carefully.
OTOH, it is possible to move away from public research and back into academia later on. (In particular, universities of applied science (FHs) like to see their staff, including professors, to have industry experience)
Now, in my experience, some groups are more "connective" than others, but I think with the substantially increased WFH it has become more difficult everywhere.
There are surely groups that are still more including with Covid regulations around than others have ever been. I cannot tell from your description whether your group is particularly unincluding or not.
More thoughts:
Will you be attending a conference this summer?
This is not only a great way to connect to people - and if your direct colleagues are not so connecting for whatever reason, you may find a group there that is more welcoming and including.
You can also try to find out how things work out at their "home base" and try to identify a group you may visit for a small "sub-project".
(You write your supervisor is nice - ask them to help with this)
Is there someone or a group of people with whom you could put in some holidays? (already 1 or 2 weeks can do a lot of good, as employee in Germany you have a right to take at least 3 weeks in a row once per year.)
Do you have a solid peer group outside work? It cannot completely substitute the lack of connectivity with your research group, but it may help sufficiently to get you along.
For anyone who has pulled through despite heavy doubts: Are you happy with your decision? Did you find a job you like? Or do you regret not quitting?
Let's put it like this: the application problem of my PhD thesis was not something I thought would "rescue the world" - although the topic [cancer diagnostics] was sold like that - what a PhD project could do (and what improved diagnostics could do to survival) was definitively oversold there. OTOH, there were "side aspects" that are much less fancy (and much less able to get one a grant) which I still hold as very important (rather basic statistics on model validation and dealing with ever-too-small sample sizes). My heart was more in that (and after I had told my professor for 3 years that I think this important, he started to say that at conferences, too ;-) ), and I learned and developed things that I still use in my everyday work now.
I also wasn't sure whether those two letters would be really that much needed. And in a way I was right: I got a job offer and moved on before handing in (many of us did). I did like science, and I stayed in research, though.
I handed in my thesis years later at a different university. I'm still not sure how much that PhD "title" helps me (though I do suspect that other people look at it more than I do). (But it's not a question that bothers me one way or the other - nor was it bothering me as much as others [institute directors/professors who paid me post-doc wages without me being a proper post-doc] when I hadn't yet handed in)
But the research experience gained is definitively valuable and important for my work.
I found a job that I like: I stayed full time in research for some 10 years, then started my own business which is still very close to research.
I don't regret anything.
I even got myself a (part time) contract again at a research institution a year ago due to the described disconnect caused by Covid rules: employees could basically choose to go to the lab, but the same institutions would not allow (or strongly discourage) contractors on premise. So I decided a side job as employee (at a research institute) would be good, and it is indeed.
No regrets :-)
