I don't know if this will help ,however I am busy replacing the belt on a Bosch tumble dryer. I assume 'wonky' means it isn't sitting straight but probably leaning towards the small pulley. My first thought is , did the suppliers give you the right belt? [Reason I say that is I had to get a longer one in spite of the one supplied is supposed to fit my WTA2002 model ] Clearly, if a belt is too short then drum will be pulled towards the drive pulley. There are various websites which will calculate correct belt length if you know 3 measurements ; diameter of big drum, diam of drive pulley and the distance between those two. From all I've read, the belt should be a little difficult to fit, but not too hard to fit. This ensures it has enough tension to turn the drum and does not just slip around it.
At the time of writing I have fitted the belt while drum is leaning towards pulley, then used various pieces of wood to lever and wedge the drum back to its correct place before fitting the back panel to secure the drum in place. I think what I've done will work.
Alan_P...
May 2019
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix tumble dryers.