Central Fl to Seattle area

Most of the route(s) is freeway, and the logical shortest/fastest.

One exception is US212 from South Dakota to Montana. That cuts a corner that I90 takes. That choice has come up often on this board. 212 is a little faster, by about an hour, but more remote. If you are at all uncomfortable taking it, stick with I90. There's nothing wrong with that choice. Sights in this area include Black Hills and Devils Tower.

Google Maps gives several alternatives, and lets you tweak the routes. This route and an alternative using I80/84/82 have about the same distance and time.

With a month, the direct fastest time isn't important.

I don't think driving weather will be problem, at least not with snow. Storms across the Plains might add some excitement. Campgrounds, though, might not be open. You'll have to check around. Opening dates for Yellowstone give you an idea of what's available above 7000 ft.

Switching to I40 across NM and AZ, and then north through Nevada or eastern CA only adds 500 miles. There might be more open camping that way.

Think of this route as a rubberband between the two corners of the country. You can stretch it quite bit either side of the direct route (the third map) without it breaking.

Just use google maps it will give you several options. Driving across the great plains gets boring after a few trips but it does give respect to our pioneer forefathers toughness.

I-90 route you may hit a snow storm early in the year, but it will not hold you up more than a day. I-90 is by far the lowest route. and least likely to get snow this time of year.

Just stay out of California. It's 55 or pay big fines

What weather problems are you worried about? Hurricanes, tornadoes, blizzards?