Sadly you are in the vanishing minority of Android users who care about this. Most people just want a phone that works. So much that many people switch to iPhones because, admittedly, many things are work better in their walled garden, and the phone is "simpler" because the OS hides many details or doesn't allow you to do anything.
I used to spend lots of time trying different ROMs, figuring out SU and SELinux stuff, and fighting with SafetyNet. These days I just use stock Samsung ROM. I still have Termux on my devices but only use them occasionally when I don't have a laptop next to me and need to do some hardcore stuff. (I might even switch to iPhone someday because the password autofill experience on Android is just atrocious and infuriating while Google has done almost nothing for the past few years.)
BitWarden on Android is pretty good for auto-filling passwords. Works in-app and in-browser.
I've been liking the Firefox autofill on Android, not sure if that fits your needs.
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There are things Android forbids you to tinker with, even on a rooted device. And it’s advertisement related things.
Personally, I would suggest trying out GrapheneOS on a modern Pixel before going to iPhone. They remove 80% of the Google annoyances and have a very good security profile compared to anything rooted and most custom ROMs that don't bother with relocking the bootloader.
You will still fail to pass device verification, but that doesn't really matter to me. I don't use tap to pay (that's why NFC credit cards are for) nor play any mobile games that actually care.
I could not imagine using a stock Samsung ROM personally, but to be fair, it has been years since I tried. Maybe I'm still just too burned from the bloatware of the early Galaxy days.