Alright jhovold here's what I did, just to make 100% I made it right:
1- I deleted my .config, GOODCONFIG, w/e.
2- Took your defconfig from https://raw.githubusercontent.com/jhovold/linux/e92057c615fec749fefcca4ab28ee5c425e3691b/arch/arm64/configs/johan_defconfig, copied that to aarch64/GOODCONFIG.
3- Then, I amake menuconfig, loaded the GOODCONFIG, added BTRFS+BTRFS POSIX permissions, re-saved the config to GOODCONFIG and also .config.
4- amake -j$(nproc), and amake modules_install
5- Copied the built kernel on the target drive, overwrote the old one.

I now see in the console logs that btrfs does load in the initramfs. But root still fails to mount. I also tried to replace the UUID= by /dev/sda3