Thanks for the response wvreeven. Short answer is I'd like to get the simulator working regardless as after tonight it looks like no clear skies for a while!
Sorry I worded that poorly. The ZWO 120 is the only camera I have right now, and it's attached to the main OTA, not the guidescope. I wanted to experiment with this and make sure I understand how everything works before I start investing in expensive CCDs or a new-ish DSLR. I have an old Nikon D40X that I intend to use as my main camera for experimentation, but as far as I can tell as it's really old the best I can do with it is an IR remote shutter release, I can't wire it up and control it through EKOS.
So my workflow (I think) will be putting the ZWO 120 on the OTA and plate solve for aligning the mount and scope (HEQ5-PRO using EQMOD and a Celestron 127SLT), park, then swap the Nikon onto the OTA and put the ZWO 120 into the guide scope for guiding. Baby steps though, got to get a successful solve before I do any of that!
At a guess, I think my shots are far too noisy and astrometry is incorrectly picking up noise as stars then failing to solve. The offline solver is basically looping through every index file and not finding a match, the online solve is also failing. Here's Capella for example:
nova.astrometry.net/user_images/3525214#original. Any advice appreciated, though with clear skies tonight hopefully a lot more time to experiment. I'll upload logs tonight if I still struggle.