besides things like missing dvb/wlan drivers and repos, but that's known
I didn't dive into kernel config yet. If there is some USB driver which you would like to be enabled, just say so (but it must be included in kernel already).
plus the mac keeps on changing...
That is easy, one line fix which will be included in next update, but since I don't have that board, you'll have to confirm if it works.
CEC seems to be a bit fragile
That's true. Be aware that Allwinner never officially supported HDMI CEC on H3. I noticed that CEC on same kernel worked with one board but not another during tests. Possible explanation could be that because there was no official support for it, HW designers couldn't test it and it may not work. Even worse, CEC pin could be unconnected or connected to GPIO for software simulation of CEC protocol.
Looking at build/beelinkx2.fex at master · armbian/build · GitHub I would say that board has CEC pin connected to GPIO port. Bad news is that CEC won't work for you for now. Good news is that kernel contains code for simulating CEC over GPIO, but I have no idea how to use that in universal way (not to break other images).
The remote I couldn't get to work with lirc
Take a look at this tutorial: Infrared Remotes [LibreELEC.wiki]