About 10 years ago, I realized that trying to train harder didn't work anymore and I had to start training smarter instead. One thing totally not about your question - I'm on Chris Carmichael's CTS email list, so I get articles maybe once a week. An interesting thing about CTS is that Chris is famous and thus his coaches are expensive and thus he attracts cyclists with money which is going to include a lot of older riders. The last article had a section saying that if you're older, like over 40 or 50 (!), and you take time completely off for a couple months say, you won't be able to get it back no matter what you do. I'm guessing that this info is not theoretical. They've seen it. This isn't directed to you, just a general observation. I had that experience maybe 15 years ago when I took a winter off and vowed that I would never do that again!
But back to your question about HRV:
I've been tracking my morning resting and standing HRs and the difference between them for decades. In my late 60s I started having trouble interpreting my data. It didn't seem reliable anymore. Doing hard days back-to-back didn't raise my standing HR like it used to - sometimes it even dropped. I had HRV recommended to me by another BF member. Being me, I researched HRV as best I could to try to find out what information I would be trying to acquire. I learned that phone apps were the way to go, so I looked at various apps to see what their ratings were and what information they produced. I settled on Elite HRV, for better or worse. Once one settles on an app, one is pretty much committed to staying with it. Continuity is a necessity because what one does is to compare today to the past. The Elite HRV interface is a bit impenetrable, but like any phone app, one just has to screw with it until one can figure out where all the screens are and what they do. It's baffling at first or at least it was for this geezer. Here's a link to a user's guide. Hopefully if one signs up, they send you a more recent one.
https://elitehrv.com/wp-content/uplo...2015-10-03.pdf
Yes, data acquisition is critical. For HRV, I use an Android phone with the Polar H10 transmitter and strap. A Garmin ANT+ transmitter and strap will also work with Android, but not with iPhone. The strap and transmitter are touchier when used for HRV than for simple HR. The strap has to be moistened and just so, and often the transmitter snaps also have to be licked as well to get perfect reception. Imperfect reception is very noticeable - you'd see weird spikes which ruin the readings. It can be frustrating. Straps get old - I think the wires corrode or something. i find they only last maybe a year, but the transmitters should last forever, though the batteries need to be replaced when the thing stops working. Any transmitter has to be paired with the phone through its Settings before it will work with an app. Elite HRV will try to sell you their CorSense pickup, but as far as I can tell, it'd be vastly inferior to a strap and transmitter, which is equivalent in accuracy to a real EKG machine
For TP, I use a Garmin 800 for cycling activity. You can upload FIT activity files from a Garmin and similar files from other devices directly to TP, Strava, etc., etc. I don't use Garmin Connect at all. TP and Strava have a list of devices and their compatible files. For non-cycling activity, I use a Polar V800 which I bought on ebay - they're still available there. The V800 device seems to last forever. Though it's obsolete, TP supports it, directly uploading from it. A complete setup includes the watch, a strap and transmitter and the clothespin USB connector. Though it might be better to get the bare watch and clothespin and buy the H10 separately, since the V800 works with many transmitters. OTOH, there are other similar devices out there, particularly Garmin watches. The V800 however has it's own built-in HRV test which I also use to do morning HR, but it doesn't have all the same info that Elite HRV delivers. So I use them both. Every morning I do the Elite HRV Morning Readiness test and then a 5' standing test, followed by the V800 orthostatic test. Totally geeking out, but getting reliable physical status numbers. Of course if you don't do training activities other than cycling, you don't need a watch like a V800.