base2 

I think your problem with a triple in the front will be the total capacity of the derailleur. As noted, you are already over the max cog size by 3 teeth.

A smaller crank set may be one solution. Another is to build a wheel around a Sturmey-Archer CS-RF3. Same ratios as a triple but all inside the rear hub. The caveat is your drive ratio is limited to 1:1 but the internal reduction of first gear lowers the realized gearing 25%. 1:0.75 is a pretty low gear.

The next question is then to cable routing. Any 3 speed mountain triple shifter ought to run it. Your road shifter won't on account of pull ratio. But it ought not be too hard to find a place to mount a shifter pod. Paul makes SRAM 31.8mm pod adaptors to mount to modern handle bars. You can also hollow out the clamp of a flat bar shifter & mount to the round part of road bar.

The CS-RF3 also comes with a cable stop meant to run full housing along the top tube to the seat stay then bare cable to the hub. As for mine, I used a pulley to facilitate routing and utilized the original front derailleur routing; Only to the pulley high on the seat tube instead of the derailleur. Then from the pulley down the seat stay.

The gearing is ridiculously wide. You sacrifice nothing and a new wheel is a lot cheaper and more efficient than a Schlumpf Drive.

But yeah. A mountain triple crank set is probably your cheapest option with a 22 or 24 tooth small ring would get you up the hills as long as you aren't in much of a hurry with that 42 tooth big ring.

Have you thought about a 12 speed bar end shifter in friction mode with a modern huge capacity derailleur? Derailleurs are up to something like 47 teeth capacity now. The derailleur won't care about the chain size or actual speeds. That's the cassettes job to worry about.