Have my reasons, mostly because all I need is local wifi lan, but does this at wifi 6 on the 5 ghz channel look about right for max speed ? 75 Mbps to 80 Mbps. Max I saw but very lucky was 90 Mbps once. It is highly unlikely to ever get that high a lot or even much. At least as far as I can tell. Unless I am doing something wrong. Would I prefer a little faster? Sure. Is it required ? Not really.
I use Beryl AX if it matters and keep it on router mode. Can not really use AP because like I was told and informed you need to have it connected to a internet source either ethernet or wifi internet source for it to work. Doubt it would help either with local wifi lan speed. Maybe I am wrong though. Do not need internet though for this that why I am only using local wifi lan.
Regarding the speed you're getting on the 5GHz channel of WiFi 6 for your local LAN, several factors can influence it. The speeds you're seeing (75 - 80Mbps, with a rare peak of 90Mbps) can be affected by things like the distance between your device and the router, interference from other wireless devices in the area, and the capabilities of your connected devices themselves.
If you're within a reasonable distance from the Beryl AX router and there's not much interference, the speeds might be close to what can be expected in your setup. You can try changing the router's channel to one with less congestion to potentially improve the speed.
As for the AP mode, you're correct that it typically requires an internet source connection. And in most cases, for pure local LAN speed improvement without internet access requirements, running in router mode is usually sufficient. However, if you want to further optimize your local LAN speed, you can check if there are any firmware updates available for your Beryl AX router.