You say you like the square format, but most portrait and landscape photos are printed as rectangles, usually in standard dimensions (8x10, 11x14, 16x20, etc.). The reason for toting around a heavy MF camera with comparatively slow lenses to a 35mm camera is the negative size. Cropping a square negative to a rectangle necessarily throws away some of that glorious negative size. It's up to you do decide if you would feel limited printing square most of the time or if it bothers you to crop so much negative out of the picture.
On the up side of square negative cameras, you never need to worry about rotating the camera or the back to alternate between "portrait" framing and "landscape" framing in the viewfinder. That also makes waist-level finders practical, which some people really like.
Macro work calls for an SLR. There's no reasonable way to do it otherwise. You'll want a camera system that offers macro lenses (flat field, close focusing) and a set of extension tubes or an extension rail.
Durability and spare parts and be approached in a couple of ways. The simplest is to buy a very common camera system. That assures the best supply of spare or replacement parts for the longest time. Hasselblad and Mamiya RB systems will have parts of a long, long time. Exotic cameras, not so much. In general, mechanical cameras are more repairable than electronic, so keep that in mind also.
I'd say the MF system that comes closest to all of your requirements is the Mamiya RB67. It has gobs of negative area (crop square if you wish), rotating back, mechanical operation, huge assortment of lenses/accessories, and are very common. I've never owned one. I own Bronica equipment, which I like very much but doesn't meet some of your requirements as well as the Mamiya. RB prices are reasonable and the only downside for you might be they are heavy. Of course, that contributes to some of their durability too.