Now this might raise a howl of protest but think on if you really have severe ankle problems.
As some of you know I had a below knee amputation and have a prosthetic which means the ankle is as good as locked at 90deg. Initially I was bu**ered when it came to wellies. I have always had Chameau full zips but the gusset prevented the boot going on.
In the garage I had a serviceable odd left boot left over from the last pair that got damaged. All i did was neatly slit the gusset and the boot, when unzipped fully, fitted easily and fold the gusset back across behind the zip as I did it up, it remained waterproof unless i actually stood in water, ok for muddy ditches etc.
When I need a new pair I shall not hesitate to slit the gusset again.
Willowbank.