It might sound silly but I've found it to work quite well for me: I store the (rather coarse) data on these exercise activities in my Google calendar.
I also do some more fine-grained analyses (e.g. looking at heart rates) via Garmin and Strava but for cross-discipline comparisons I mostly rely on my calendar entries.
I've found the calendar to be extremely reliable -- knock on wood -- over long time horizons (I have exercise data going back to 2016). Also, it's very flexible. I store additional metadata, e.g. the distance of a run, in the event description.
This is pretty much exactly what I've been wanting to do for years. Love the calendar solution.
Still not sure I understand how you're getting the data off wearables and into the calendar events, though (manual? custom API calls? Garmin's API isn't very friendly for extracting metrics).
I've been a Garmin user for nearly a decade and would really like to get the data off their platform so I can do more with it.
There are a few self-hosted options (https://github.com/tcgoetz/GarminDB seems like the best) but I keep kicking that can down the road.