hmm. When I was only shooting film (because it was the only thing available ... yeah, I'm old), on a long trip like that I might once carry my Nikon with a typical four to five lens kit (from the 21, 35, 50, 85, 135, 200 lenses I had) and practice changing films mid-roll so that I could use different films as the whim took me. I might also take a compact (Rollei 35 or Minox 35), and/or a MF camera. In later years, when most of my long trips were by motorcycle, I'd carry just one camera (Nikon FM or Leica M) and two lenses for lack of space. Even later in the film era, I'd just carry the Rollei 35S and/or a Minox B or C submini.
Once the digital era was upon us, I was well in tune with carrying one camera and two/three lenses (wide, normal, short tele or some subset) as the entire kit. One trip I also carried a Polaroid, another trip I carried my iPhone and the Polaroid, another the iPhone and a 6x6 folder, etc.
As time has gone on, I've carried less and less and done what feels to me to be more and more satisfying photography. Because, to my mind, what makes the difference is removing distractions and options, leaving just what your vision is and what you come across in your travels to focus on. But I'm moving into a philosophy of photography there, and that's really up to you to meditate on and decide for yourself.
😉
I'd say, one medium size backpack with a GA645 + 10 rolls of film, an M with two/three lenses and 20 rolls of film, a Contax T ... is going to be one very full backpack. A Rolleiflex MX or 3.5F isn't so much heavier but it is pretty bulky to carry, will take up a lot of space. So I'd say ... pull out your backpack and start trials by seeing what fits and what it weighs. That will determine the physical limits of what you can carry that way. THEN, knowing that, think carefully on just what photography you want to do, and how much of it needs what films, what lenses, what formats... and soon you'll have your personal answer to this question.
I think sometime soon I'm going to do an entire trip with an iPhone, a Polaroid SX-70, a dozen packs of film, tripod, tele lens, flash, self timer, filters ... That already just about fills my medium backpack.
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It will be fun.
😉
G