[Xiaomi Mi 11] 1lopes:
ok, good points u got there

seems like connectivity doesnt matter to you, i can see, even though it has more recent tech. if its the case or something else that you are really fine with then youre good

you also would like to take photos by being more than average (id suppose some landscapes, sunsets, structures, monuments, etc?). the mi 11 offers a great sensor and a camera system, and you love to take good quality photos. if i put into perspective that maybe you just are a overall photo shooter, then it is just enough *

i never had a snapdragon 888 powered device, but from some sources the snapdragon 888 can withstand normal and daily tasks, but on heavier tasks it can throttle (thats the issue. when the socket reaches a high temp or overheats under heavy tasks, it can max out or temporarily decrease the potential of running at full speed and try to slow it down, getting you a decrease in performance till it cools down. from the web, it seems like it overheats easier, but keep in mind i am completely unsure and never used a 888 device)

since you dont game, id see this problem not occur frequently and you should be fine on daily tasks.

i know about MIUI and its low points and cons. miui seems to be a ruby at first touch, but forget about it after some months from my experience with 2 xiaomi devices. aosp is great to try out the max of the device potential speed (i never saw the difference between MIUI responsiveness till I got AOSP on my redmi note 9 pro)

the battery should be fine. my note 9 pro with an 5020mah battery that holds ~65% health gets me thru the day with 4h sot. i wouldnt see the Mi 11 have such a bad battery, specially if you are looking for a second hand, those should be above ~80% battery health and not reduce by alot.

regarding your doubt due to the socket affecting the battery, yes, the socket throttles, uses more battery as it runs hot and on its high speed. as it is hot, it might heat the battery by a bit too, affecting the life of it and the longevity whenever it overheats, reducing its lifespan and battery health. not by much, if it goes underneath on this situation for just minutes. but remember, if its only for daily tasks, then dont worry about it alot, shouldnt occur frequently in my opinion, and it wont get hot that high.

* now, if you really love taking just some average photos like those, and would like to have 108MP shots and other modes, then moving to AOSP can be a issue, since some of AOSP ROMs dont offer MIUI camera built in. ive done a bit of research and saw some maintainers build it into the ROMs, so it should work.

the reason that this is important is that, in my personal opinion, having a GCam port working for your device is kind of difficult, due to the tons of XMLs, versions, types of GCams everywhere, takes a ton of time to set up to your needs. if you dont care about it, then maybe a regular XML you could find would suit you.

a recommendation from my perspective could be an S21 series, specially the S21+ for a closer budget, and being released 1 year later than the Mi 11.

it offers about the same specs, only the camera being 64MP, but offering an telephoto lens including 3x hybrid zoom (yes, 64MP with 3x zoom), and a big difference in the screen sadly.

the perfomance is at the same level of the Mi 11, offering the snapdragon 888 on US models, or the global exynos 2100 (now, the big issue is, if you have ever gotten a samsung device running exynos that would easily overheat, then rip, because exynos sockets overheat alot easier lol, but should be average for your daily tasks to be honest, since it only happened alot more frequently in older samsung models of mine)

anyways, to its screen, it offers a much lower resolution (1080x2040 to 1440x3020) and PPI (394 to 515). as you referred, its a big strength in it, so reconsider it if you are already thinking about the S21.

now, in software level, samsung already offers good enough OEMs that there is basically no custom ROMs there. people adore the OEM nowadays by samsung (personally, me too as i loved custom ROMs, with my s10e too (well, kind of, before a one UI 5 port).

the stock os from samsung should fit you just fine in my opinion, but if you really love AOSP, then get the Mi 11. *you should also get the Mi 11 since Samsung camera app cant be built to AOSP ROMs, so you would lose the camera functionality without the use of appropriate GCam settings

so, if you prefer a good OEM software, but a lower screen quality and camera, then go for the S21+.

if you love an quality screen, better camera system, but concerned about MIUI, possibly keeping the MIUI camera on AOSPs, go for the Mi 11.

i hope i helped, sorry for any mistakes and if u dont understand anything just tell