Thanks for your detailed reply.
Is he a pet dog or a working dog? (Fur, feather or security work)
Sounds like he has been conditioned to be constantly doing high energy tasks and hasn't been taught how to simply relax.
If he's a pet dog, why is there a focus on a high level of cardiovascular fitness and scent work (finding a hidden ball)? Keeping a dog extremely fit and constantly stimulated without training control doesn't 'tire him out' it just makes your dog a stronger idiot....
Enrichment exercises don't have to concentrate on high energy stuff. It can (and should) involve training the dog to respond to commands (sit, down, come, away, for example). Get to training the dog to use his brain more than his physical ability. Do It with food, not a toy (toys are stimulators, food requires cognition and a calm attitude to digest).
My dogs are fit. They work. But the first 6 months of their lives focussed on making a steady dog. I have fox red and black labs, all from 5+ generation working lines, health tested, blah blah... they can hunt all day in all weather. But when I tell them to wait, they wait. And they'll wait for hours until I send them. I taught them to have impulse control. They want to work, but they can control themselves emotionally until I send them. At home they lay on the floor and sleep (even on days off). If I get up from the sofa they'll raise their head to see what's up, but if I don't say anything they stay where they are. They know they're not working, nothings gonna kick off.
My point being by explaining all of this is that my dogs work hard, but they're under control, and they know when to switch off. Sounds like your dog lacks control and doesn't know when to switch off. Every engagement it has with your girlfriend is high energy without asking the dog to calm down and respond to command. At home he demands attention (and probably gets it every time) and doesn't know to settle. im not digging mate, just my opinion.
For what it's worth, I also have collie at home. He's from farm stock. Very intelligent. He's 10 now. Never worked him in his life. He gets a walk in the morning and a walk in the evening. No special food, no toys, no enrichment. He's a pet for my Mrs. Never had a behaviour problem with him ever, because from 6 weeks old I trained him to do the basics (here, down, come, away, no) so he has good manners.
Apologies if none of that makes any sense. I'd be happy to arrange a call or video chat to discuss privately if that helps?