A small update to my post from yesterday:

I did one final (really, the last one!) optimization to my network (luckily without new hardware). Instead of letting the new ASUS router do all the networking and QoS, I repurposed one of my GL.iNet Beryl AX routers with vanilla OpenWRT as the main router, doing SQM, DHCP, DNS proxying, and all the other stuff. Now the ASUS is running in Access Point Mode, handling just the mesh WiFi and partly acting as a LAN switch (because the Beryl AX has just one WAN and one LAN port). My bufferbloat is much better (or non-existent), and the Cloudflare Speedtest is showing me “Great” in all categories, even when connected with WiFi to the repeater.

I think I will stay without cake-autorate, but try to tweak the optimal SQM bandwidth limits a bit over the following days.