Reboot into new OS image Remotely

I have a few old rpi 3b+ still running stretch which i would like to upgrade to either bullseye or buster. They are all remote and i access them via AnyDesk or reverse ssh tunnels as a backup. I have attempted to simply upgrade into buster/bullseye on a rpi at home, with the same setup as these remote ones. My hope was that i would upgrade, then reverse ssh or AnyDesk into it after the upgrade, but they never managed to upgrade properly (attempted about 6 tiems with multiple variations and always failed for one reason or another). I can't risk not being able to remote login, so i'm looking at other options. Ideally here is what i'd like to do: 1. Remote log into my rpi. 2. Download a pre-configured image of buster/bullseye (with all the things i need, Anydesk etc..). 3. Reboot the rpi into the newly downloaded OS image. I've looked into using some multiboot tools, but they generally need you to do all the setup on a separate SD ard / USB etc. I need to be able to perform all these remotely via AnyDesk or ssh, and on the SD card that contains the currently running OS. Anyone able to give me some pointers?