Take a look here. The plain vanilla 19.07/R7800 build with that script, software offload enabled, and no SQM can easily do a single stream 700..800Mbps NAT-ed (LAN <--> WAN, iperf3) as per my test just a couple of days ago. Wired, not wireless. hnyman's build will do just fine as that build is not making any code changes.
If that does not help and nothing else is hogging the CPU (can you post the screenshot of top -d 1 or htop -d 1) then it is either a faulty router, wiring, or one of the computers are not fast enough....
UPDATE: the tx-usecs changes get lost after every change via LuCI, so a reboot is required to get those back. Can be done via hotplug.d, but I never cared to optimize that part.
Yeah, and it was caused by me. So, a false alarm.