Ideas for May trail 200-300-ish miles, Western US?

At that time of year, a chunk of the Hayduke, GET, or AZT seems reasonable. Also, have a look here for some other routes –  https://simblissity.net/get/

Although “only” 150 miles long, you could also check out the Kokopelli trail from Fruita, CO to Moab, UT. It’s mainly a mountain biking trail, but folks do hike it.

The Owyhee Canyonlands section of the Oregon Desert Trail is usually snow-free by May:

https://onda.org/regions/oregon-desert-trail/

If you decide to venture further east (a lot further) the Benton McKaye Trail in GA/TN/NC is 300 miles.  The Long Trail in Vermont is probably a good option too.

The Hayduke is pretty rough requiring caches iirc.  The GET seems a bit more doable, though parts of AZ closer to Phoenix would be pretty hot by then (most of AZ is in a low desert while NM is high desert, so the former will be hotter in late spring).

Depending on the weather maybe the AZ line going into the Gila area of the GET.  Alternatively taking the CDT from Lordsburg to the Gila and maybe back is something to consider?  Not sure if there’s an easy exit going north or east on the CDT/GET going out north of the Gila though.

If next May is warmer, consider an “upper New Mexico loop”

Beautiful ideas – thank you, and keep them coming! Definitely am now considering Oregon as well as an out & back on Mogollon rim trail.