Some spouts screw on using tapered pipe threads, some slide on and have an O-ring to seal to the OD of the copper pipe with a set screw to hold the spout in place. I've used both. The O-ring variety is easier, IMO

A properly prepared and soldered joint is quite strong. I've tried to unscrew a copper threaded fitting that ended up twisting and breaking off the copper pipe without disturbing the soldered connection. (This was an expansion tank fitting, not a bathtub!) Make sure things are firmly attached inside the wall - use a drop ear elbow that is fully support and securely fixed to the structure. And, you don't need to screw the threaded spouts on super tight - the spout connection doesn't have to resist 100 psi continuous pressure; the pressure it experiences is intermittent and low. Fill the threads with pipe dope and just screw it on until snug and you'll be fine when/if removal time comes.

Just use a tub spout elbow from the mixing valve, not a drop ear elbow that will have another connection.

You can test for leaks at rough-in, then once your surround is done, you just cut it off the proper length. You don't have to worry about a MIP adapter or threaded stub length and maybe unknown surround thickness.

Then you can use a slip-on spout, much easier. You don't really ever have to worry about your soldered connection at the mixing valve (or even a male doped fitting), it's the internal pieces of the spout that would ever sieze from long term leaking and mineral build-up, but those are all pretty much plastic and need teflon on any threads anyway.