I spend a fair bit of time looking at random seeds online and I have a few for you with frost caves close to the spawn.

the above mentioned HHcLC5acQt is a very well known "good seed". plenty of caves. I actually have my 2nd vanilla game on it.
CCzzNkYznS this one has a small island just north of spawn that has a mountain that's silly with caves. the map itself is also pretty damn good for a no sailing play through.
4RfxnFvfFy the current best map for known-map speed running and also has loads of caves near to spawn.

as for the mistlands, it's best to simply not sail out too far from the starting island. once you get close enough, a mistlands will render with nothing in it forever. that is until the actual update happens.