Did you also ask the “go to” HRV-app makers?
eliteHRV, HRV4Training, Whoop, Oura…?
Most people I know use one of the above and usually sync to TP/WKO. I know that elite and HRV4training can push to TP. It’s a one time fee for the user with elite, not sure about HRV4. Maybe those guys could provide the same kind of integration with intervals.icu?