OpenWrt support isn't quite there yet for the GL-AX1800. When gl-inet says it "runs on OpenWrt" they mean they modified OpenWrt to make their firmware.
What maurer said....
If you absolutely must stay under $100, a NanoPi R2S (get the metal case with it to keep it cool) plus a used all-in-one that you can use for its WiFi and switch should stay in your budget and comfortably meet your performance needs. The NanoPi R4S is well worth the extra money for future proofing, but if your budget is fixed, the R2S will do the job (~80 Mbps OpenVPN, ~300 Mbps Wireguard, ~450 Mbps SQM Cake).