Enabling 256-QAM in ImmortalWrt didn't help though. So, it either doesn't work correctly in ImmortalWrt or the issue is elsewhere. But I was eager to test it, since the first thing I noticed when trying the GL.iNet beta firmware was that my link speed had increased.
I think the slower 2.4 GHz (802.11ax) speed observed when using some Apple products could be a separate issue, much like it was for 5 GHz. However, I can't prove it since I don't own any Apple products.
I used the firmware selector, selected a snapshot and didn't keep any settings when installing the firmware. And everything seemed to work, but I reverted back to OpenWrt once I was done testing.