If you were using Android I would suggest just go for some mainstream model from Samsung (S series), Xiaomi (Mi X) or Pixel (basically any model) with pretty mu

If you were using Android I would suggest just go for some mainstream model from Samsung (S series), Xiaomi (Mi X) or Pixel (basically any model) with pretty much guaranteed long term unofficial ROM updates even after official support cease if your problem were depreciated new app versions, but since you are voluntarily locked yourself to closed iOS, I have no suggestion other than stop updating apps and use the apps which were fine before. Btw. multitasking doesn't really work even on brand new iPhones. For the record I am running gapps free Lineage with Android 8.1 on my phone from 2018 without any issues, I can hardly find app which would not run on 5 years old Android 8.1, if something (rarely) doesn't work it's because of relying on gapps.