id be thinking its the frame too except the only pivot that had play i just replaced the 2 bushings in it, i assumed that was what was causing the issue in the first place but nope :D i just faced the caliper bolt mounts and nada either. I didnt spot any cracks anywhere in the frame but il have another look to make sure.
Im not sure the cassette could be causing the issue as it happens any power down on the pedals, i can literally drag it while walking and get the vibrations, ill check either way just in case though, could be anything at this stage.