So You’ve only changed within your contract once before? The focus to the kuga.
Then you renewed with the Hyundai Tuscan, changing as it’s heavy on fuel may cost you £250 but you are within your rights to change if it isn’t suitable and the running costs are now effecting your wife’s health. That will be change no 2 but should be close to or past the three year limit if I’ve read your post correctly.
Also there are grants available if you can’t afford AP’s and meet the criteria, ask motability for help in applying.
Do you have any idea what car you would change to?