I don't understand what you mean. Aquasuite X73 release notes do not say there is a firmware update for the Aquaero 6 Pro, so I don't understand what you mean by "none can upgrade firmware on X.73". When you download Aquasuite. its just an installer. My understanding is that the installer looks at what Aquacomputer devices you have and the status of your license. It will install the latest version (currently X.73) if your license is active. If your license is expired. it will install whatever version was the latest at the time your license expired. This ensures that you can always download a version of Aquasuite even if you choose not to keep your license active.

This has nothing to do with firmware updates, but if a new version of Aquasuite does include one or more firmware updates, once that firmware is updated, the updated device(s) will not be able to communicate with earlier versions of Aquasuite. Version X.73 included a firmware update for the Leakshield only. I looked at the Aquasuite release notes. It looks like the Aquaero 5 & 6 firmware was last updated to version 2104 with Aquasuite X.12 in February 2020. Aquasuite X.12 was a major update and included firmware updates for the D5 Next, Quadro, Octo, Farbwerk360, Farbwerk Nano, and Aquaero 5 & 6.

Also, are you saying that you have 5 Aquaero 6 Pros? I see they are in stock in the Aquacomputer online Shop, I think for the first time in a very long time. Did you just buy them?

We release today a Bugfix Release that fix this issue. Wait for The X74 Version.
Update the aquasuite after the relase with the setup from the website, then update your devices.