I recently published the first complete version of a project that can help with creating activities with Unity. If you want to check it out here's the repository: Dissonity
This file contains information about my experience using Unity with the Embedded App SDK, so if you plan to make a Discord activity with Unity you may find this helpful. I am not a professional developer though, and the SDK has become public just recently, so take my words with a pinch of salt.
I used Unity 2022.3.8f1, I haven't tested other versions but it may work similarly.