I'd like to try Lineage OS and others like it, but the bootloader unlocking process not keen to go through again (like I used to a few years ago) + I've not seen a single dedicated custom ROM for the Redmi 9T (not the Note 9T), last I looked ....
..... so don't want to brick it for no good reason while it's still pretty smooth & not causing me problems + always unsure if banking apps, for example, will work on a given custom ROM.
I hope mine lasts a year to two more (have had it around 2 years), but the 9T is notorious for "dead boot" - well documented in the relevant GSM Arena thread.
At least apps get updates, including security, for browsers (Brave, Chrome etc.), so though I'd like system-wide security ones, I won't ditch the phone because they've stopped (to now at least).
EDIT - looked at that GSM thread for 1st time in a while & someone commented on lots of ROMs available through Telegram, but I don't even use it so will continue with stock - less hassle (to me) all-round.