Oh boy, EKDobbs, you're gong to love this: Look, kid, I spent the first six years of photographing, meaning from 14 to 20, with a 35mm SLR and a 50mm lens. Period, that was it. Won awards, was published, did pretty well thank you. Until you have a specific reason for a piece of equipment- meaning a client or a project demands it- focus on prints, on results. I assume that you are scanning? Make prints of the scans, any way you can.
You want to try 6x6? Mask your 645 viewfinder- simple pieces of cardboard on each side. That will tell you 80% of what you need to know about the square format. You are young, so your eyes are good and you'll be able to see the 4.5 x 4.5 screen pretty well.
Take that Mamiya 645 and drive it into the ground. Work that thing every which way you can. Do portraits, landscapes, interiors, sidewalks, everything. Make prints one way or another. Put them on the wall, pin them up. Use a hunk of the garage wall if that is all, basement wall. Look at what you are doing. LOOK!!! And look at other people's work. At museums and books, not Flickr. Look at videos and films, see how they make a shot work. Magazines. Go to the local library and ask if they have any photo archive materials. See what got saved, see if they are of interest, try to understand why and why not.
Here, look at this series. Looks to me to be two lenses- 'normal', about 75mm for your Mamiya, and 'wide', about 50mm. It's got it all- landscapes, interiors, space, not bad for some guy shooting one film, one body, a couple of lenses. Buy the medium-wide lens foro yur Mamiya first, and then you could do what this guy did-
Or this guy, again very limited equipment-
Or this guy, maybe two lenses, all 35mm-
Or this gal, one lens-
Wait until you have no choice but use another camera. Until then, focus on shooting and looking at your own work and other people's work. Beg or borrow other equipment. And save your money for travel with one camera rather than staying home with ten cameras. There's always time to get another camera, but there isn't always time to be be in your late teens exploring the world in all its richness.