Adventure Cycling - Southern Tier - is clearly the first choice - - but
(Saint Augustine, Florida to San Diego, California)
https://www.adventurecycling.org/rou...southern-tier/

If you have already done it -
I rode a little north of that in 2008 -
From the Sea Islands of Georgia to Del Rio, Texas
https://www.crazyguyonabike.com/doc/...c_id=3402&v=Jp

If there is any way possible - start on Sapelo Island - a magical place.
Jekyll Island is mega touristy, golf, condos, etc.
The counties of the Black Belt - because of the soils - are majority African American.
These GA, ALA, MISS counties are some of the poorest in the U.S.
Limited services - esp. camping - - but worth it.

I do not understand why the Southern Tier skips the Cajun country of bayou Acadia in Louisiana.
Stunningly beautiful, struggling after multiple hurricanes, incredibly friendly & generous.

There are 5 bazillion ways to cross Texas besides the Southern Tier.

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On other trips I have taken US 62 in West Texas via Guadalupe Mtns NP - very remote.
US 380 in southern NM is also an option - linking to US 60.
The latter is further north - can link you to the best stretches of Old Route 66 west of Flagstaff.
(Seligman-Kingman; Kingman-Needles; Goffs-Ludlow)
But the weather in March can still be quite wintry - esp. at higher elevation.

Just some ideas. - - J

Photo - Sapelo Island Live Oaks