As of writing, Rocknix is not officially supported on the Anbernic RG DS, a budget handheld that really needs an optimized OS. (GammaOS Next also works but performance is better on Rocknix - however you can do multi-tasking on GammaOS like on foldables along with some other features.) The following guide shows you how to install and use it.
Credit to Retro Game Corps for their guide,
I would not have figured out I needed to configure extlinux otherwise. Also credit to
the ROCKNIX team for developing the OS! (And half credit to Anbernic cause the handheld
is alright but omg the performance...)