Hi KP, welcome to the forum.
There are so many factors to consider when you have a desire to make a long distance relationship become more. Even if you and M were both single, it would be complicated enough. If you are seriously considering moving to CO to be closer to M, I'd ask, what about job opportunities for you there? What about leaving your friends/family in Florida? How would that feel?
If you've only met his wife twice, you and she need to build a solid relationship. There might be jealousy issues on both sides. Does she have another lover/lovers as well, or is she monogamous? Do they have children? Do you? How would you combine families?
So, when you say you and M have been romantic for several years, do you mean you two have sex, and the wife is fine with that?
When you say you are going to see him again soon, will it be in CO, or Fla? Why not meet him on his home turf and spend the day hanging out with both members of the married couple?
Of course, if and when you do decide to move to CO, you should NOT consider just moving in with them. Get your own place, get to know the wife. Get to know M in his own environment. This "sister wife" thing could take a lot of work to negotiate.
Love withers under constraint; its very essence is liberty. It is compatible neither with envy, jealousy or fear. It is there most pure, perfect and unlimited when its votaries live in confidence, equality and unreserve. -- Shelley
Mags (poly, F, 60) loving miss pixi (poly, F, 38) since January 2009, living together since 2013
also loving Punk (monogamish, 42, M) since Oct 2015
"Master," (mono, 34), miss pixi's Dom for 2 years