Not sure I can be much help as I am at a similar stage and have a similar problem with C7-G7.
I've been stuck at around 55 changes for 3 weeks now.
However this morning I did try a different method of changing. Previously I had been twisting my wrist to make the change from C7-G7, keeping my fingers in roughly the same position. This meant that the change felt uncomfortable, and my 3rd finger ended up in the middle of the E string fret.
This morning I tried a different method - from C7, I led with my 3rd finger, 'bouncing' fingers 2&3 up 1 string each and just dropping the index down one. This feels much more comfortable, and I think will speed it up.