How to install TWRP after Magisk?
There is different method from Android 9 and later.
For Android 9 and 10 devices, ensure to disable dm_verity by flashing an empty vbmeta before flashing magisk patched boot. Some devices most especially those using Unisoc chipset require that you generate your custom signed vbmeta.
Sorry, I don't know more but only this difference about Android 9 or newer.