Add a second tray/location showing the still-running background applications (as opposed to the paused/killed ones that you can still switch to). Then the user

Add a second tray/location showing the still-running background applications (as opposed to the paused/killed ones that you can still switch to). Then the user can quickly see what's taking up resources and pause or kill it themselves, optionally moving applications between the unkillable and killable locations. No need to dive into per-app settings except maybe to determine the default behavior. You can do this dynamically based on which applications are running that you'd like to keep running (or not). They could even justifiably restrict this capability to the M-series iPads given the substantial performance improvements (including efficiency cores) compared to the older A-series iPads if they wanted to create motivation to upgrade.