I think the guarding is a symptom of something else here, and the first and most obvious thing to check is pain.
Unfortunately that is not easy and you will need a vet who is VERY up to date on the relationship between pain and behaviour, I would go with a veterinary behaviourist and there aren't many in the country. My top two would be Amber Batson and Kendal Shepherd.
Speak to your own vet about the possibility of a pain medication trial to see if that improves things (it can, but its a bit clunky, however if theres nothing else you can do at the moment it isn't a bad option assuming your vet is on board).
Unfortunately not all pain will respond to pain medications and not all pain medications are suitable to such a trial.
I'd also want him seen by either a Ttouch practitioner* or a veterinary phsyiotherapist, at home.
*Ttouch is a load of woo, however their practitioners are extremely well educated in canine anatomy, correct movement and signs of pain, if you can't see a vet physio, see a Ttouch person.
Unfortunately with the best will in the world, most vets will struggle to see obscure pain, soft tissue pain etc within the confines of the average veterinary practice as dogs tense up and hide things, and most vets simply haven't the space to let a dog move around and behave naturally, hence seeing someone at your house rather than at a vets office. If your vet will come to you, great... they are likely to see much more, but they may not be able to do this.
I would get as much footage of your dog as you can, walking, trotting, moving around, in the house, in the garden, on the lead, so that someone can have a look (or ideally several someones), but of course it would be better to have someone in person if you can.