I came into the github not expecting to like this, but you've done a great job so far! I currently use a combination of Garmin Connect (track on watch & head unit), Intervals.icu (for fitness/freshness and more obvious tracking of performance improvement over time via various power & HR metrics), Strava (for segment and route performance change over time, and of course the social aspect... and routing), Komoot (only for cycling routing), and Elevate for Strava (just testing now, but it looks like a crappier version of intervals.icu so far), and Smashrun for running-only tracking. Oh, and Veloviewer for cycling-only performance/history and route visualizations.
It's a lot.
The problem is that there isn't a single place where you can get both the holistic metrics and the plan in a way that most users will want (which almost mandates a Strava interface). Apps like Veloviewer or Elevate already have this, but both either fall very short functionally or specifically only target a subset of features.
When I complete a run or ride, the first thing is to check Garmin to see how it related to, or impacted, my physiological health metrics, since I rely on Garmin for things like sleep quality, training readiness, and load tracking. Then to Strava to see how it went compared the previous times on the route and look at segments, or perhaps to give kudos to someone I was with. Then to intervals.icu to dig into the performance metrics (power, HR).
Intervals.icu is great because it handles workout planning, too, but the one big thing it's missing is routes & segments for IRL activities. Veloviewer has this, but it is missing all of the training/planning/fitness features. Imho, the killer app in the space is going to be whoever is able to augment the planning/tracking pieces, which are largely grunt work to develop, with the social bits that Strava has. The existential risk for indy devs is that your product is just a feature for Strava, and you could easily become disintermediated (or they shut down or charge exorbitant rates for API access -- it's already highly rate limited).
You're doing great work, but I'll keep paying for intervals.icu, Strava & Veloviewer for now because they all do slightly different things, even if there's significant overlap.