I don't think any TLR can substitute for an RZ67, unless you were totally not using the RZ to its full potential. If you want to keep the flexibility of lens choice and therefore buy a Mamiya TLR, you're back to the space+weight problems again; and the lenses, while decent, are still not nearly as good as the better RZ lenses (I own both systems). If you buy a Rollei, it's nice and small but quite inflexible.
If you can't afford a Mamiya 7, I would strongly suggest instead that a 6x6 or 645 SLR might be a better option. You can even get autofocus, lenses as fast as f/2, real closeups with no parallax crap, and some are smaller and lighter than a TLR. While 645 is less negative area, that doesn't matter if you habitually crop to a rectangle, e.g. for 8x10 or 16x20 prints, and there are some very small, modern, light and fast 645 cameras around, up to and including digital.
A 645 SLR could be smaller than a Rollei TLR and retain most of the flexibility you had with the RZ67, but with 60% of the negative area. If you were not shooting the finest-possible-grain film (Acros, TMX, Efke-25, etc) in your RZ, then you weren't getting the most out of it anyway. Shoot film in the 645 that's a stop slower and finer-grain (TMX instead of TMY, FP4 instead of HP5, etc) than you did in the RZ, open the aperture up a stop and you'll get the same exposure speed, similar DOF and just as good print quality as you were getting before.