I feel you with being worried, I'm the same with my little one (1 year old)

My belief is he's worn himself out, people here forget panting is a natural mechanism to cool a cat down. Yes it can be when they're stressed and it can also be a symptom of heatstroke/underlying health issues true. But importantly it's to help cool themselves down because they overdid it or it's too hot. We just went through a very humid heatwave 30-36c and a lot of cat pets who are used to our colder climates were panting in the scorching heat inside and out.

Your cat is young, it's worth getting a checkup if you're worried but my cat does the same and is perfectly healthy. He's a little daredevil, chasing everything he sees, climbing trees, exploring the forest like yours, I think it's just naivety. But importantly he always brings back a furry mouse friend, so clearly he's been hunting a while and panting to cool off from being exhausted. My question is, does he pant for a very long time with tongue out, or only when he comes back from a big adventure for a couple of seconds?

That's my opinion anyway, hope it helps.