802.11ax Routers

No, you will have to port OpenWrt support for your device yourself; the ipq807x target support is still under development/ pending (see the Xiaomi ax3600 thread for details).

If you need a solution now, get a different device, otherwise be prepared to spend significant time and efforts on doing the necessary development (and hope that ZBT didn't shut the door too closely via secure boot).

I've got RT3200 on a sale, but its range is worse than old Archer C7 v2. Is it only this Belkin/Linksys issue? What would have similar or better range than that Archer ?

I found a KuWfi Tenbay WIFI-6 AX T-MB5EU-V01 used for $10. Used the TFTP method to flash OpenWrt, not issue with it so far.