Hi ThatDummy,
I hope your guitar learning makes nice progress? :-) It might be indeed a nice combination together with modular. The modules you can't get in Eurorack, perhaps you can get them as guitar effects pedals! :-) These two worlds together should give you even more endless possibilities :-)
I would like you to refer to my comments I just made yesterday to a very similar case and question. Most of my comments there apply to your rack as well, so please follow up on that advice there. You got already a mixer I see, so that's fine but you are still missing an audio interface; however you might be able to solve that with 1U modules or the rack where they fit in.
For details please refer to my reply to Bvkuz also in this sub-Forum "Racks" under the post name "My 1st Rack - Starting point". For feedback, comments, questions, etcetera, you can then use this post to continue.
Good luck with the learning curves (both instruments, guitar and modular) and kind regards, Garfield Modular.
For review reports of Eurorack modules, please refer to https://garfieldmodular.net/ for PDF formatted downloads