Vdsl 100mbit wdr3600 and sqm?

what do you guys think about solution like this? https://www.apu-board.de/produkte/apu2c0.html

is it worth to buy such a device and are there known wlan ac nics which works flawless on linux?

Yes its look like so. Strange. We are using the 4040 at Freifunk without the problem you have. Maybe is your unit broken?

CPU power and price ratio is at my taste a bit far of my chart.

apu will do.
but still, for this money you can get something more standard/commodity and less limited.

celeron should be enough.
laptop-class or low-end desktop cpu with a mini board should also not pull more than 10W idle but does not struggle as much when working gbit sqm/nas/vpn.