Just a quick update on how i resolved my freewheel search. I wanted to keep it vintage in both look and spirit, so we went with a vintage Shimano 6-speed MF-Z012. It's one of the freewheels as called out on the tech doc shared earlier that is compatible with the thumbshifter. So we should be good to go.

And btw i couldn't agree more with the look of the new shimano freewheels with the writing on the granny gear. Horrid. Thx everyone.