Hi
@MickW
Welcome to the forum
I'm a retired Orthopaedic Nurse so may well be 'out-of-date' re some of what I learnt but perhaps I can offer some aspects for you to consider or discuss with your GP/Orthopaedic surgeon.
Clearly, you are considering surgery and I assume, from that, that you experience a lot of pain, mobility issues perhaps, and maybe back issues due to your resultant posture or how you have to move in order to get around.
So, this is a bit lengthy but, I consider, vital information to help you to a faster, safer healing in the long run, with minimising complications - and pain etc.
Any surgery will have potential healing delays with a diabetic, but you can improve your chances considerably by having the best control possible. The best thing you can do after lower limb surgery (whether diabetic or not) is to be sure your leg is elevated (fluid will only run downhill - you could use a spirit level if unsure

) as you really need to minimise swelling. This elevation really does help the wound to heal the fastest, due to minimising the fluid / swelling in addition to consequently reducing the risk of infection - not to be taken lightly.
So, foot higher than knee and hip - for at least 2 weeks

During this time, you'll be encouraged to be up to the loo, to get from A to B within the house etc but the essential thing is elevation 99% of your day - lots of reading, resting, chats on the phone, meals being put in front of you etc etc etc Make the most of the time to catch up on all that reading etc
A fusion will most likely considerably lessen your pain, as there is then no joint to move, or totally remove it (from that joint) It's also possibly the surgeon may be able to correct any deformity, to a large extent, but nothing is guarranteed, of course.
However, a fusion is very 'final'. Now, it never seems to be mentioned but I do wonder how much strain/torque is then put on adjacent joints, causing problems in the future. Would you be able/prepared to discuss pro's and con's with your Consultant/Senior Registrar?
You haven't mentioned it but has an Ankle joint Replacement been suggested? If not, it could be the Ortho Surgeons don't feel it to be a suitable option for you, or could be something to do with it being a No-No for people with diabetes. I really don't know if diabetes has any particular considerations re joint replacement surgery. Although ankle replacements aren't yet as brilliant as hip replacements tend to be, and certainly more complicated to do I understand (and imagine) they are still, very often, extremely satisfactory. Also, a joint replacement can, at a later stage, if necessary then be converted to a fusion although I would imagine there could be some shortening in leg length as a consequence. If an ankle joint hasn't been mentioned or discussed, I would personally want to consider it as an option, be given some facts, given the option to research, think and discuss it before making a decision. We are all entitled to an 'informed' choice and not to just accept what we are told.
I wish you the all the best for whatever your future holds. If you'd like to PM me for further discussion, please don't hesitate. Also, please remember I can't give 'advise' as to what you should do but I do feel I can suggest things to consider and follow up with asking / discussing with appropriate medical staff.
Julie