I haven’t played much with 32bit grbl, but you might find grblhal interesting:
I believe it supports most of the arm mcus found on the popular skr boards. I haven’t researched further than it’s propose and existence. So I can’t offer much help getting that working.
On the fluidnc side there are several options but all esp32 based:
At the bottom of that page there is a ramps adapter board that looks like it can be built on a budget. Fluidnc is for sure easy to setup and use. I’m curious what is the cheapest fluidnc board that can run say a dual endstop primo, with basic relay spindle and flood/mist.