Michael, just for context and insight into options, does the AI have one or two inputs and if so are they XLR, 3/4" jack or combo? That would influence what you can do.
I am not a Mac user, so have no idea of how audio devices and routing work on a Mac. That said, from what I can recall of what people have said in the Community, I think you have the native capability to route signal paths, more than is native on a Windows PC.
If that is so then may be it is possible to create this path: guitar->amp->Mac (via USB?)->GB. Vocal routed via AI into GB and balancing levels should be possible pre-recording the performance. Once balance then route GB to OBS and record live.
Again, no Mac experience but assume OBS on the Mac should work similarly to how it does on the PC.
I am an OBS user and I make live multi-track recordings with it. By live I mean all the mixing to set levels and apply effects is done pre-recording and there is no post-processing.
Based on my experience you should be able to set up OBS with the vocal audio from the AI and the guitar via the USB input. While there is no post-processing in OBS you can control the levels of each input source in OBS to achieve balance. You can also apply effects using the OBS filters that can be applied to each audio source. OBS has a few default filters as well as accessing VST plugins that you may have available on the Mac.
How best to record the acoustic? Via the amp to apply some effects is an option.
You can mic the amp. I think if you control the amp loudness then you should be able to get good results with either a standard condensor or dynamic mic. I think (not an expert) if you were wanting to record an electric guitar playing through a tube amp cranked then you might need a dynamic instrument mic designed to handle the sound levels.
If you have a second mic and the AI can accept both then another option would be to use a condensor to track the acoustic guitar and a dynamic vocal mic for the singing. I say dynamic as it gives the best results in terms of limiting guitar bleed onto the vocal track. Then you can add effects and balance levels in GB. You could do that pre-recording setup and route to OBS live or record audio dry in GB and OBS as your record. In the latter case you can mix the audio in GB after recording and replace the video audio as recorded in OBS with a processed audio in a suitable video editor (I assume iMovie would enable this).
And it sounds like your guitar is electro-acoustic, so again if the AI has the right inputs, you could record the acoustic direct into GB or OBS via the AI. From a guitar tone perspective, this would probably deliver the poorest results unless you have really good quality electronics. You do have the option to use effects in GB to improve the sound of the guitar recorded direct.
Apologies, I know quite a lot to digest, and I may miss the mark either through not understanding the Mac world or something else contextual.
Anyhow, hope it makes sense and is helpful.
I prefer the approach I described above using OBS without a DAW in the chain. Once familiar the setup pre-recording doesn’t take long and then there is no post-processing.