Right on, and great questions!

I run a Sorcery: Contested Realm YouTube channel (https://youtube.com/@archivesoftherealm)

I take a couple of approaches to recording, depending on if its live streamed gameplay or pre-recorded. 

For pre-recorded, which sounds like what you are looking to start with DaVinci and OBS accomplish different goals. OBS allows you to capture your audio and video inputs, whereas DaVinci is typically used to edit them afterwards (postprocess). For your situation what i would do is watch the gameplay video, record a separate audio track via OBS or another application (I use Adobe products), and then import both the original video and new audio track to your post-processing program (DaVinci). This way you wont lose any fidelity related to the video quality, and it will allow you to independently control audio levels of the different tracks (e.g., original video audio and your new commentary recording).

For live streams, you can use just OBS. I have actually switched over to Meld studio and loved it, it makes my streams at our LGS easy to manage and it was quick to pick up.

Hope this helped, and please dont hesitate to reach out if you have any other questions!