Screen burn in is not a thing on phones in my experience, though a few say they have experienced it. I had a Samsung Note 3 for 2 years, a Moto and my two Xiaomi amoleds now for 3 years.
I'm fairly sure Xiaomi give a 2 year warranty on Redmi here in the UK, but I can't find proof right this moment.
I take it you are in a different world region. Do you have a local Mi. com or Xiaomi. com site or Mi Store app. They do fair to very good daily and weekly discounts.
The A77s is not sold here. It's probably good enough. It has a 720p LCD / IPS screen which is fine, but the 1080p Amoled with eye care features seems a lot better, plus the newer SD685 chipset etc
I can only soeak from experience, what I've learned and what I read but I think the MIUI experience is very good and at least equal to Samsung"s user interface. I never signed in to their own Mi Browser and only recently used their Mi Cloud storage allowance.
There are short tutorials and videos on where to turn off Ad Preferences in various areas (maybe 4 or 5). I uninstalled unwanted pre installed apps like LinkedIn, Facebook etc and my system storage is very low.
I think that's the same with most Redmi models.
So I'm selling the Redmi big time!! it looks like.
Update bugs have not been apparent with mine, and I personally wouldn't consider it a concern if I was buying a new Redmi.
The Redmi Note 12 4g is currently only selling a 4gb ram / 128 gb storage version from their store currently.
If you can get a 6gb version all the better, but 4gb wouldn't trouble me for my uses. Does the A77s have 6 or 8gb of ram.
Check out the Spec sheets of both if you haven't done (GSMArena or Phone Arena)