Installing minecraft on a raspberry pi 4 ... it would be easy they said. But ok, it seems to be working now - custom launchers, textures, xbox accounts. Another maze solved!
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For reference, MultiMC launcher solved most of my issues. Together with the Fabric mod.
Obviously, minecraft for raspberry pi worked straight away ... but that was not what my 11 year old was after. The whole Xbox account thing was ... being persistent. But we now have a family account, with permissions and everything.