I turned on orbot and did NOT configure it to run in VPN mode and then I configured amethyst to use tor (on port 9050 which orbot is bound to by default) and it's working perfectly. None of the other traffic on my phone is going through tor.
Any app that is proxy-aware could send its traffic to port 9050 to use tor without the rest of the traffic on the phone using tor.
Vitor Pamplona (npub1gcx…nj5z) thanks for making Amethyst usable over tor - that's a great feature.