Yeah, the functionality of the CSM is going to be entirely dependent on how the boardmaker implemented it in the BIOS, and they seem to be very inconsistent (often even between boards from the same manufacturer). You can play around with the settings a bit more, but not having Secure Boot shouldn't be a big deal unless you're planning to run Windows 11 soon.
If your Displayports work when inside Windows, you could also just.....disable CSM and go back to how you were running, unless no display during boot bothers you (I'm sure it's a PITA when you need to go into the BIOS, but that shouldn't be often).
The good news is that whenever you decide to upgrade your GPU, it will no longer be an issue with the newer card.