@dc2.0 I think it ‘may’ be working. I tired the 26″ holes again; it works on diagnonals, rather than bottom holes and top holes, so it was definitely in the right ones in the 26″ setting. The wheel definitely didn’t fit the first time I tried on the 26″ setting, but I wonder if it was because the hybrid tyre had a taller profile compared to the slick turbo tyre. It wasn’t the easiest but with some technique it fitted in. So it’s secured by the QR now but the clamp at the back cannot be used as the wheel is too close. However I wonder now if that was only for the larger wheel size/setting, but turning the wheel behind the clamp for fine adjustment meant allowed a small turn and it seems to be calibrated using the app.
I have to admit the Tacx instructions are extremely vague but maybe the tyre size / not using the lever combo has done the trick. Zwift now seems to give with virtually no effort (are zwift puns allowed?) 220 watts and I believe 2.4w/kg ish with a quick 30 pedal.
So user error/ bad instructions aside I think it is working. Thank you for everyone for their input, very much appreciated!