Pure maths "dropout" wanting to finish MSc thesis
You have a lot of advantages, actually. First, though, ignore the age issue. It isn't something you can change and time moves on no matter what you do.
You seem to have contact with appropriate academics in your field. You can probably exploit that. But, since you are a bit non-traditional, for someone to take you on they might be concerned about whether you would be a burden or not.
Let me suggest a path, though there are others, certainly. Find a project that you think has merit in your field. Write up an outline of a thesis and fill in some parts of it, such as the literature search. Include a few ideas that might be developed into theorems, but which aren't currently known (yes, hard, I know). If you can't do that much, at least name one or two &quot;threads&quot; that might be pulled.
Take that to some professor that you have contact with and ask whether they think this has promise, first, as a bachelor's thesis. If yes, then ask whether they will guide it and get you over that first hurdle. It might be that a second thesis can follow the first along the same lines. Spell out your academic needs and background as you have here. A period of burnout after intense study is pretty common and many of us will understand that and have possibly experienced it.
But the outline will show someone that you are serious and the existing personal relationship will assure someone that working with you will be relatively easy and not a waste of time.

Even if the professor doesn't normally teach at the B.Sc level, they might be willing to work with you if you make their task easy. Note that I'm assuming that the bachelors thesis is an absolute requirement that can't be avoided. If so, tackle it first, even if your paper is more advanced than would normally be required.