Oh okay wait, this may have turned into a bug. I can use the wacom wheel to adjust brush size which is just too clunky to be practical (if the tablet is on a desk) and in fact the first time I did that, the ability to use pen button/drag briefly returned. But only once, and after I painted a stroke click/drag seems to have magically shifted to altering the opacity instead. I can't seem to toggle that back to brush size. The wacom button is currently set to right click but even if I set that to middle click the opacity still gets changed.
Thanks.