How much of it is actual training? And how much is just snuffle matts and food related 'lazy' games?
I'm asking because we very often see people on mumsnet, and I see people who come to our rescue asking for help, who say they do lots of mental stimulation with their dog but actually 99% of it is doing a snuffle mat or hide and seek with treats. What all these people have in common is they are doing this sort of stuff with their dog and it's incredibly over-stimulated and chronically bored. None of that, except the actual 'training' is mental stimulation...it's over-exciting your dog and giving him food = energy. It works for a few dogs, but not many. You're not doing scent work.
At our rescue (Golden Retriever specific) found that very often, when these dogs are relinquished and they go into foster and the 'brain games' stop...the dogs calm down. It's like a completely different dog appears within a few days/weeks maximum - because they're no longer over-stimulated by these games, and instead are given proper training. We actually say on our local breed website that these games are a really bad a idea - for all the reasons laid out above.
I have Golden's so slightly similar. By 11 months, mine are getting about 90 minutes exercise a day but it's all off-lead. About 8(ish) miles. I think very often people fail to differentiate between 'walks' and exercise. A slow sniffy walk is all well and good, but you have a high-energy breed slow 'sniffy' walks really do not cut it. Just because a dog is show line doesn't mean it's exercise needs are not significant.
So, 'mental stimulation' should mean training - not food related games, which often do more harm than good - and exercise should mean decent walks - at his age, I'd be expecting 6/7miles a day minimum of good exercise.
Sadly, based off what you've said, your dog sounds like it's been given far too much 'energy' and not being given significant outlet. And I'd hazard a good guess he's planting on a walk because he's not very well trained and is incredibly bored/his energy needs are not being met. You say he has 'recall when no distractions' and 'settles well when knackered.' That's not a well trained dog. That's a fair-weather dog.
Personally, I would cut out all those games and do 3/4 5-10 minute training sessions a day with your dog. I'd be giving him proper walks, not just sniffy ones, and making sure that all his needs were met. You also need to teach him to settle without entertainment - part of your problem is I imagine you give him these 'games' to entertain him, and thus he's never learnt to settle.
None of the above is meant to be nasty or rude, but it's based off years of experience dealing with situations like this.