After conducting further research, it appears that at least some people have successfully fitted the Rival AXS eTap derailleur with a Garbaruk cage, making it work with an 11-50T cassette. My plan is to start by using the SRAM-compatible version of the Garbaruk cage and 12s normal chain (YBN SLA e.g.). If that doesn’t work as well as expected, I can still try switching the pulley wheels.
What I'm still unsure about is if I should choose a dub or dub wide crankset and which chainring would be appropriate.