ollee + CR2032 mod
as the title suggests, i managed to cram a CR2032 in an ollee modded F-91W. There is a guide on how to do the battery modification online. However, this leaves the piezoelectric element unconnected and therefore the watch is not able to make any sound anymore. I also managed to fix that. I will say this required quite a bit of finicky work (also because i had to find out how to even do it along the way) and even some soldering, but it's definitely doable. Furthermore, a future battery change is now even easier than before because you do not even have to remove the movement module anymore. Just drop in a new battery after unscrewing the back.
Hopefully I won't have to do this very often though ;)
My first original battery (that came delivered with the watch) died pretty quickly, which inspired me to do this.
If anyone is interested in repeating the mod and has some questions, feel free to ask.
TL;DR: the watch now has about 2-3x the battery capacity and everything still works.
EDIT: I posted pictures of the internals in a comment below