I would look for a bike with a top tube that you can clear as captain, then try an extra long seat post for the stoker. Some seat post flex is actually a desirable thing on a tandem.

A stoker stem like this: https://co-motion.com/product/m
with the addition of another standard stem of the right length and angle ought to be able to get the stoker’s bars in a reasonable position.

Would be be great if you could visit a bike shop that sells and knows tandems. If you want to stay in the used market for cost reasons, just be prepared to do a fair amount of wrenching on the bike yourself.

If tandeming ends ends up working for the two of you, all the effort will be well worth it.