Alright, you might not like this.
The question "How can make the experience of an online talk similar to the experience of a blackboard talk on-site" has the same answer as the question "How can I make streaming a movie online similar to the experience of watching a play in the theatre?":
You simply can't.
A movie is not a play and a screen is not a theatre stage. Both media have their advantages and pecularities, but they are too different to replicate the typical movie streaming experience on a theatre stage or vice versa. Same for an online talk and a blackboard talk.
(You might be able to rescue some of the blackboard spirit if you just give a blackboard talk and stream it via a camera and a microphone. But it will still not be the same experience, and it only works at a reasonable level of quality with the right setting and equipment, which you might or might not have available.)
You say you like blackboard talks (so do I), so let's see what makes this medium so well-suited for math talks; we can check for each point to which extent it can be replicated online. A (probably non-exhaustive) list of relevant points is:
Pacing. Writing things on the blackboard limits the speed at which we can discuss material in a talk. Many people seriously need such a limit (as you can see by checking what they do if they use slides, where no such natural limit exists).
This can be replicated online by writing by hand on a tablet computer instead of using text that has been written before.
Alternatively, you can use a medium without such a limit (for instance, slides), but force yourself not to put too much material in it. This is absolutely possible if one is really committed to do so.
Temporal synchronisation of speaking and writing. When you write something on the board, you will typically talk about what you are writing right now or about what you have just written.
Again, this can by replicated online by writing via hand on a tablet.
It can only partially be replicated by using pre-written text that is revealed piecewise as you give the talk. This solution is imperfect (and requires much more work to prepare, in particular if you want to do it really well, i.e. not only reveal things from left to right and top-to-bottom). It should be noted, though, that some people do not like such overlay mechanisms (link 1, link 2). Some other people though (including myself) get seriously annoyed if such overlay mechanisms are not used (e.g., not using them has typically the consequence that I am, as a member of the audience, unable follow which points the speaker is currently talking about. Using somekind of pointer to resolve this does typically not work well, in my experience.)
Geometrical synchronisation of speaking and writing. At a blackboard, the speaker will often be physically close to the written content they are currently speaking about, and increase the distance now and then during some elaboration that is not written on the blackboard. A good and experienced speaker can use the distance to the board to direct the audience' attention and to modulate the "density" of the presentation (e.g., move away from the board and insert a number of less technical sentences now and then in order to give the audience time to relax mentally a bit).
I see no way to replicate this online.
Using gestures and movements to keep the audience engaged. One great things about blackboard talks is that somebody is really moving at the blackboard. Personally, I find myself much more motivated to listen carefully if I see that there is something going on phyiscally and in three dimensions.
This cannot be replicated online. Putting your camera on during the talk, so that people can at least see you talking, is probably better than nothing, but very far from the experience in a lecture hall or seminar room.
Ability to correct a mistake. If you write something on the blackboard that turns out to be wrong, you can easily correct it.
This can be replicated online be some media, but not by others. Generally speaking, writing things on a tablet computer by hand gives you the opportunity to correct things. Whether pre-written text can be fixed on-the-fly depends on the details of the technical solution that you use.
Keeping large parts of the text available during the talk. This is a point that you mention as very important in your question. I'll argue that it actually consists of two important points:
Availability for the audience: As you say, somebody in the audience can just look at another part of the blackboard to recall what, say, "Assumption 1" was. The main point here is that it typcially takes very little time (and thus distraction) to look this up.
I claim that this can be replicated online only to a very limited degree. The only thing that you can probably do is, as you have already suggested, to send the entire document to your audience before the talk.
I think there are two major caveats, though: (i) Looking something up in those notes will often be more distracting for someone in the audience than it would be one the blackboard (because they might have to scroll through the document, and if they use only one screen, then this will cover the talk itself while they do so.) (ii) I strongly suspect (without having checked empirical evidence for it, though) that people are more easily distracted during online talks anyway, and this adds yet another distraction.
Availability for the speaker: This refers to the possibility that the speaker can refer back to results, assumptions, properties, formulas, and so on, from previous parts of the talk.
At the blackboard this is possible since everything (or much of it) is still there; it is also easier to follow since the speaker can move (phyiscally) to the point of reference and point there also physically, for instance with their hand or a stick (just in case that somebody finds the idea of a stick "old-fashioned": if something is located to high for your arms and you have the choice between using a stick or a laser pointer - please use the stick, it is so much easier to follow than those tiny points of light.)
I'll argue that - and that's the part that you might not like - it is not possible to replicate this online.
Going back in slides is, as you have already observed, extremely confusing. If you use slides, don't do it.
If you use a longer document - say, one which you write by hand via the talk - and scroll down during the talk, you might try to scroll up to refer to something that you have written earlier. I have done so in the past now and then, but my impression was that it does not work well during lectures for students - and it does not work at all during research talks, since the density of the material is typically much higher there.
So the situation is from my perspective as simple as that: if you give an online research talk, you are not allowed to go back (no matter which technology or software you use). If, say, "property (a)" is no longer on screen, you are not allowed to refer to it; instead, you have to put it there again (either by writing it by hand again, or by revealing a prepared version of it). This is the only way to ensure that many people in the audience will be able to follow you.
(In case that this seems like an exaggeration: no, I'm dead serious - Don't go back!)
Bottom line. An online talk is not a blackboard talk, and you won't be able to generate the same experience for your audience.
So rather than trying hard to achieve something which is not possible (with a high risk of messing it up), I suggest to choose an online medium which you think is most suitable for your presonal way to give a talk, and then adjust your talk such that it works well with that particular medium.
