The biggest bang for the buck would be to connect Apple HealthFit to intervals.icu.
FitnessSyncer and HealthKit both can do it, they support a million sites, if the API is in place to receive those it shouldn’t be to hard to convince them to add I.i
I also use HRV4Training from the brilliant Marco Altini, and I’m sure he’d probably add it, but direct export form HealthKit would cover all the iOS users.