I've been training with HR for ~20 years. I hear what you're saying. I do hill repeats on the same hills, so either time or how high I get when the interval's over tell me how I'm doing. I also use Strava now, which gives me some data: estimated watts and VAM particularly. However by far the best thing is to have a group ride with the same bunch of folks every week. Of course as summer comes you'll all get faster so there are confounders. However over the years, that's been my best source of "am I getting better" week by week. When summer comes here, there are big rides I've done every year so those really show me if my training plan has given good results this year.

I guess one might say, what are you training for? And are you getting better at doing that thing? That's how training's always been. You want to drop that rider good, right? Or maybe just hang with them? You gotta figure out how to do that. When you can, you've gotten better. One year it was learning to recover from successive accelerations. Turned out I was better at that than some riders. Another year I learned to hill sprint, dropped everyone. All that focus on numbers sometimes gets one separated from the real objectives. As they say, in winter train your weaknesses, in summer your strengths. Figure out your objectives and train to accomplish them. HR isn't even necessary. I ride with some fast guys including an age group national champ who ride and train with no instrumentation at all. It's not necessary, distracts, and adds weight. A lot of getting faster is simply riding smarter.