Just a data point or two that I have not had this problem of apps being killed in the background, not in A14 or now in A15 (upgraded two weeks ago). I did disabled many unneeded apps using adb when first got the phone (sorry, don’t have the list now), and have also set the screen refresh rate to 60Hz. My wife’s FP5 is still running A14, and she does not have the problem either (though I did the same initial setup to her phone too). One thing that DOES happen in FP5 running A15 is that Vivaldi browser sometimes reboots. Not often, but still probably related to the problems reported here. Hope this helps.