I'm aware of the gl.inet devices and maybe I will try find a suitable one.
And thanks for the two other recommendations.
I birefly checked the Zyxel M1: Is that single core MIPS CPU way more performant than the CPU on the WNDR 3800? Do you know any Wireguard performance figures for those devices?
Thanks for the hints so far.
Availability in Germany seems to be a bit of a problem. Some devices were probably never sold on this market here.
Zyxel seems to be available, I will try to find out something about Wireguard performance.
Here is a quick update.
I received today a new Zyxel Multy M1 for 30€ plus delivery, so a really good price. They are sold in packs and the person selling it was not in need for it anymore.
I setup OpenWrt on it (23.05.0-rc3) and ran a test via Wireguard.
Throughput it much better at appr. 140 Mbps. But I assume the bottleneck now is the Fritzbox 7530 serving as the Wireguard peer. htop shows free resources on the CPU.
I will repeat some tests some days later on a different Wireguard peer.
Anyway, the speed is already very fine for the intended purpose. Wifi seems to be working well at a quick glance.