They collect minor things like search terms and app usage info. The rest is stored on the server only if you explicitly upload a photo or a recorded track or something similar. No collection of location data or WiFi networks etc. The telemetry can be blocked of course if you have such setup (VPN+pihole for example)
I have Dynalink DL-WRX36, running openwrt since day 0. Iirc it was 60 euro year ago. Everything works, wireguard too. No complaints. I believe there is openwrt stable build for it already, though I an still running snapshot as I am too lazy to update.