Tough question. I've also been trying to make possible retirement plans though I've still got a few years to go.
There's so many options, but you would have to weigh the pros and cons of each. The midwest is nice. A bit of mid 19rh century history and lots of old towns and farms to hit. But the winters can be long and brutal. There's also the problem of having way too many anti detecting regulations and outright bans in local areas.
Florida has potential, but brutally hot in the summer. Lots of hunting pressure and some odd regulations about water hunting especially around leased water areas near wrecks. But after big storms I hear that the whole area changes and new treasures could pop up most anywhere on the beaches.
The East has more temperate weather and abounds in CW era history. Most areas are pretty reasonable for cost of living.
The Northeast of course is Colonial hunting grounds. Best chance of finding the oldest stuff in the country. But cost of living is high and winters can be brutal, depending on the area. Tough landscape around mountains, but best chance of finding undeveloped land with colonial activity.
The Southern states have a lot of history as well. Good chance of finding pre CW items there.
The SW states tend to be arid and preserve coins and artifacts better. But there is so much land that also may have never seen the footprint of a human. But here, and in the West you have gold country. Detecting for nuggets, panning, sluicing, etc can net some funds for the hard work if you are lucky.
West coast area has less history overall, but I've seen mining camp hunts that turned up lots of gold coins and relics from the days of the '49ers.
Good luck and happy hunting.