I'm very behind on updating this thread due to our break in Wales and a mad week beforehand, but a few things of note:
The weekend before we went away, we had a busy weekend. Sat morning we went to the vets for Bear's checkup and Monty's hydro / Librela, then the afternoon we went to visit our old neighbours in their new house and huge garden.
On the Sunday, my MIL and partner came over for dinner, the first time she's visited our house since the Christmas before last. In summary, Bear was occupied the whole weekend by other people / dogs, had loads of exercise, and didn't try to mouth me once in the two days. Once he'd run off steam, he was really good around our visitors (apart from a Jam sponge cake incident on the Sunday which I shall skim over

) and had loads of fun with his doggy friends running around their garden .
Monday, all was back to normal, just me and dogs, and the mouthing resumed. So, I'm beginning to wonder if he does need more stimulation, not less? Or just a more interesting array of people to be with instead of me and OH?
Slight aside: At the vets, we spoke about the issues we have had with him getting fixated with other dogs, and the possibility of Suprelorin. After a full examination, the vet pronounced him in excellent health (I asked him to check his teeth well just in case there was anything giving him toothache, but as expected they were declared perfect) but said he may well be a good candidate for Suprelorin as he's now classed as a fully grown adult dog. I'm really pleased we weren't pushed into castration at all by them, as happened at a different vet with Monty. The only downside they pointed out was that the implant is as expensive as castration. If the results are really prominent, then like others have done we will probably go with castration further down the line, but if it's minimal impact we will just have to ride out the remaining teenage months (years? I saw the teenage phase being described as up to three years on a training page the other day

)
I'm going to start a diary soon - but the big change here is that OH is going to be at home every day from next week, and I do expect to see some immediate improvement as Bear will have someone else to be with during the day, not just me on my random tea breaks. OH is going to walk them separately as much as possible, but just keep one day per week with our dog walker so they can have a group walk with other dogs occasionally.