Just a FYI with regards to :GX90#, getCommandSingleCharResponse will break that call since it returns a function call based on a double return, not an integer. It's a bug with the current. Tried to make a pull request for Howard, but messed up somehow with github's stuff so I just sent it to the list.
I've got it called with the (new) getCommandSingleCharErrorOrLongResponse which has a short timeout (200ms), and if for some reason it isn't working, I've got it falling back based on :GU# which may not be correct if a custom rate has been entered.
And FINALLY, I think I have everything where status gets set. I just had to override/replace/catch so many things, so I may have missed them. Last one I think was manually doing EQUATORIAL_EOD_COORD which was getting set to IPS_BUSY (aka goto) when called in (base) Telescope. Which I think finally kills the issue of KStars/Ekos detecting the end of a slew. (I hope anyway.) I added some things to kstars as debug/try to fix, so double checking with a clean copy that those aren't the only reason it works. (None of it was triggering those changes.)
I got so used to the Blur-Beep, with the default sounds (slew started/sync. That my mind is now expecting it while testing out the alignment. LOL, It happened right as I was considering if to post this, or wait, expecting things to go bad. Fortunately, The cause wasn't OnStep though, Ekos initiated a Meridian flip which caused the dreaded (by me at least) "Slew detected..." message.
I haven't tested all versions again, but so far it seems good, so I'm declaring victory for tonight.