Speed is not the issue here, being bound to the same 5GHz channel is.
You don’t want to use 2.4GHz wherever possible because of how crowded that band is.
What I want is 5GHz on different channels for client and AP side and there is no single GL.iNet device that offers this. As you said, you could connect two devices, but that would just complicate installation and administration. The clean solution here is getting a device with true independent radios, like the MikroTik devices I mentioned.