For me going back to twrp browsing files and flashing stuff still works. So there seems to be no encryption issue for me. It's strange that it doesn't work for you
yes I got it from here https://forum.xda-developers.com/Mi-9/development/recovery-unofficial-twrp-xiaomi-mi-9-t3905825
And everything works fine. I flashed Magisk as always after booted to recovery
Not tried to add a new one yet but the old ones which were set up before the upgrade are still present. I did a dirty flash and only wiped Cache and Dalvik
Upgraded to Android 10 on my Mi9 with cepheus twrp, wiped cache and dalvik after flashing Rom and rebooted. After that booted again to Recovery and flashed Magisk. Everything works fine and Storage is still encrypted.