OK I've now tried the older driver v5.21.01. Using this did indeed restore the 1x and slower slew speeds as triggered from the PC – kind of.
I've alternated 5.30.10 and 5.21.01 a few times, and I'm getting erratic results with both. In the newer version, the slow slew speeds never work. But in both versions, sometimes a button press will do something different for the first press compared to the second. E.g 12x may look like 1x at first press and then be back to normal. Also, sometimes a control seems stuck – a brief press causes a slew, or a focus change, to continue as if the button were continuously pressed, and it can only be stopped by reversing the action.
All this is with my AP1200 GTO3, firmware V2. I also have an AP900 GTO2, firmware E. I've had no trouble with this with driver 5.30.10. So I don't see how this can be an issue with the latest version on older mounts.
I am controlling both through long serial cables connected to serial to USB convertors. I have several different types of these, so I'm trying to test if a faulty convertor could be the issue.
My friend also has an AP1200 GTO3 firmware V2, and says he is finding no trouble with the latest driver.