I also had a bad vibration problem and wheel balancing made little difference.
I too used the spare method and mine turned out to be offside front tyre causing the problem. Swapped out and all good now.
Its a pain to check if it turns out to be the last one checked but cheap to try.
Also check if one wheel is hotter than the other just after a run in case you have a sticky caliper, sounds odd I know but experienced similar vibration on an old Mazda xedos 9 I had.
Good luck with your diagnosis.