Have you tried Fennec? It's a stable and up-to-date release of Firefox with full support for add-ons and about:config
Not all add-ons work for mobile, but many of them do. You just have to use the "save to collection" feature to add them. I've got sponsorblock working in mine.
See, I've heard people say this, but this has not been my experience at all, and the data seem to support that it's not slower on Android. If anything, it should be faster.
I always disable intrusive settings like "suggestions from sponsors" and "improve the Firefox experience," so maybe that's the reason?
Could be a unique hardware thing as well. I have an S21 FE running the stock OS, but I've disabled certain system apps I find annoying, such as Samsung Internet and Modes and Routines