What problem are you trying to solve by ditching the rear derailer? IME rear gear changers work better than front, especially with triples. The added loss from 2 pulley wheels versus 1 is insignifcant.
If I were building a bullet proof trouble free ride, I would start with an old Shimano 600 6 speed cassette hub, the original version with flat cogs and a screw-on 13. Those are light, the axles are supported at the ends for maximum strength and make a 6s wheel with about the same offset as a 5s, which is way better than any modern wheel. You might even be able to modify it for even less dish if you replaced the 13 with a simple lock ring, or maybe get more gears by using 8s cogs and spacers. Use it with Simpex retrofriction downtube shifters. IF you only want 6 gears you don't need a front derailer at all, which mostly eliminates chain drops. Or you could go really old school with 5s wheel on 120mm spacing, and use a SunTour Ultra 6 freewheel. Wider spacing results in too much axle bending with those old freewheel hubs. All of those parts are probably more available than a 2 speed freewheel (which I've never seen) or even Biopace chainrings.
Hope this helps.
em
Last edited by eddy m; 10-01-22 at 07:52 AM.