You can probably just dedicate 1GB for the framebuffer, and then let Linux dynamically allocate memory to it at runtime. As far as I can tell this doesn't impact performance, so there's no downside. (Older AMD stacks required a static partitioning under Linux, I think).
I haven’t had much time with it, but I’ve had to set the split in the BIOS. There’s probably a way to do it from within Linux though. Also hoping some progress is made on using the AMD NPU in Linux. I know it only recently got kernel level support.