To answer a few question: MBase128 could be used instead but only Japanese games supported it directly and only certain ones like Private Eye Dol would be practical for swapping the whole 2k bank from internal Duo to the base. There's also Tennokoe card which can swap but it is only 8k (4 banks) and in all of the cases you would either need guide or Japanese reading skill to figure out how to use it. Plus those requires CD so if you have PCE, Coregrafx, or TG-16 without CD dock, you're out of luck.
My hack does not require learning another language at all. Just solder in new chip, switches (or my ATMega with LED readout), and it's ready. Still working today after a few months.
Replacing the original SOIC is the hardest part. Tennokoe2 and I believe TurboBooster Plus uses DIP version instead and would be easier but this assumes you are not going to use CD system at all. For Duo, all seems to use SOIC and that requires very fine desoldering skill or a good hot air rework station.