Rock box is definitely a DIY phenomenon, developed in peoples’ spare time. Usually by the time a port is ready for daily use, the player has been discontinued.
However, there is an equally robust DIY community for upgrading more popular players, like the iPod 5.5 & 6. With Rockbox, a new battery, and an adapter from iflash.xyz, you’ll be set for a while.
All that said, you can still find the AGPTEK Rocker new on Amazon. The port is very useable.