rpi3 image which is
aarch64 based and has uboot+mainline kernel is the supported one.
rpi-3a+ is also
aarch64 , why you are telling it is not?
if with the latest kernel from the offical rpi will help the rpi-3a+ boot then problem solved, right?
or if some one work to boot the rpi-3a+ in uboot then the board will be supported by manjaro arm?
I do not build the raspberry pi kernel for aarch64.