Waltly does have Z-couplers, I have a hardtail mtb frame inbound. I’m sure they are a clone and not really a Paragon coupler. Paragon makes steel and ti couplers under license from Santana, any builder can use them on a non-tandem bike. They should be retrofit-able, but obviously you’ll need to refinish/repaint the bike. I have 3 Waltly S&S frames (road, gravel, wife’s gravel) in the house and have never had one loosen, but typically after the first ride a bit of tightening is needed (maybe 1/4 turn). Walty’s lock ring that holds the coupler is steel and can rust, they’re sending me stainless replacements with this new frame.
My Slim Chance has a Z, and it’s pretty much the coolest thing since sliced bread. The frame uses a Ritchey-style seat tube coupler, those Zs are expensive and the seat tube system is rock solid.
https://mamilmisings.com/a-slim-chance-at-success