Forget about phone HRV apps. There is no need to invest a 1000 $ for the top of the line Garmin Fenix 6X pro solar with chest strap. I bought my girlfriend a much cheaper Garmin Venu for 150 $ without chest strap and it has almost the same capabilities and software as its bigger brother. Including an oximeter and alarm function in case of a fall. And if you are consistently wearing and using it the software knows for years how you trained and how you slept. No need for manual input. And if you are not prepared to invest in such a technology then why did invest in the Mountain Training Group a bigger amount? The UH site and the 7-hour video tutorial will give you more info to start with. And the coaches can not help you if your metrics is wrong.
I have been using for 20 years top-of-the-line Suunto watches and I miss the real-time EPOC calculation on which the Training Effect is based. But Garmin is superior in regard to software integration. Both use in the background the Fisrtbeat algorithms for HRV analysis developed by a Finnish institute. Hope my advice helps!