Frequent quick breathing

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Hi everyone! I hope this is the right place to post this but if not, please let me know. I adopted my first border collie two months ago from a humane society in Arizona, a 3 year old BC (neutered by the shelter). 

What I’m wondering about: He breathes really quickly when he’s resting, fairly often — up to 80 breaths per minute. He doesn’t do it when he’s deeply sleeping at night, first thing in the morning, or all the time (sometimes it lasts 20m, sometimes hours) but any other time is fair game and it’s pretty consistent once it gets dark. Occasionally it accompanies anxious behaviors, like licking his lips or panting, but most of the time he’s just lying down next to me while I work or even falling asleep. I took him to a vet when I first adopted him and showed her a video of his breathing, and she said she wasn’t concerned as it didn’t seem like he had any trouble breathing - he was just breathing quickly. He tested negative for heartworm and is on preventatives, and was treated for an ear infection that I took him back for a check-up on recently and has fully resolved. No other issues detected with a physical exam. It doesn’t seem like he has vision problems as we play fetch outside in the backyard at night with no issues.

I thought maybe he was anxious because he was understimulated or bored, but he doesn’t exhibit any negative or destructive behaviors and he’s engaged with puzzle toys / snuffle mats / chews / structured walking lessons / obedience training / backyard fetch for quite a large portion of the day (see context below for more details). While I’m sure he’s still also adjusting to his new life, he’s bonded really closely with us and seems like he’s very happy and grateful overall. 

We then thought maybe it was separation anxiety after searching this forum because we often stay up outside past his bedtime and he stays 2-3 ft away from us at all times when he can, so we tried zen chews (didn’t see any real notable difference) and tried putting him to bed routinely around 11pm without us, but he was very reluctant to go into our bedroom once he realized we weren’t coming with him. The other factor is that it does happen during the day even when I’m with him (though maybe less often when my partner and I are BOTH with him — hard to say). 

I’m a bit stumped — is this even a problem? If so, I would really appreciate any thoughts from all of you who know much more about BCs than I do — both about what it could be and what I should try doing to address it. If context would be helpful, here’s a bit about him and our routine:

His background: He was picked up as a stray, but he doesn’t seem to have a high working drive or stamina (he gets tired playing fetch at full speed after 10-15 minutes), was somewhat underweight, and seemed like he had never been in a house (and definitely not on a leash!). I don’t think anyone had ever trained him and I don’t think he was working — he doesn’t demonstrate many herding behaviors — but he picks things up quickly. It’s hard for him to refocus or pay attention to me when he’s stimulated by anything like birds / bunnies / strangers / smells / new things, so I don’t think he was trained to have a human partner. He’s extremely attached to me and will immediately follow me if I move more than 6ft away in the house; he doesn’t seem visibly stressed when he’s alone and will settle quickly (and doesn’t whine or bark or pace), but he does try his best to follow me out the door. He’s extremely friendly and good-natured but hasn’t seemed like he’s met very many people before, so he gets very excited (but friendly) when I take him out around strangers. Aside from wanting to follow me outside, he has zero behavioral problems in our home that weren’t fixed with a gentle “ah-ah” and a day of positively reinforcing the opposite behavior.

Our routine: I leave him alone with full rein of the house and backyard maybe 4-5 hours a day for 1 hour increments while I’m in our front yard just so he won’t expect us to be home 100% of the time after quarantine. I play full-speed jumping / fetching with him in our backyard until he’s worn out (it takes about 15m) twice a day (I’d play with him more but he doesn’t seem interested until evening-time). I take him for walks once or twice a day, though we’re working on leash manners so that may be more frustrating than relaxing (lots of stopping and starting). We work on tricks maybe three times a day for 10-15m increments, which he seems to love, and I give him his daily food in snuffle mats or puzzles for him to figure out. He gets various things to chew on throughout the day. We’re very affectionate to him throughout the day, and he falls asleep in our bed (but moves himself to his crate about 6 feet away during the night).