Good question, I’ve pondered the same thing with regard to climbing/trails/etc.
I’ve always done a “ramp test” (which I got from Jack Daniels or Joe Friel I can’t remember which), where you perform a series of 3min speed increases on a treadmill and capture the HR for each, then stop and stand still for 1min, capturing both the average and the final BPM at the end of that minute. Add up all the averages and you get a “ramp score.” It’s applicable only to yourself of course but you can gauge improvement that way at least as the number drops over the course of training.
When I was doing a ton of climbing races I even converted it into what I called the “Ramp Ramp Test” where I did all the same speed-increments but I did them at 12% grade on the treadmill.
I’d be curious what the experts here suggest for tracking progress.