- Pulse Ox blood saturation
- Incident detection and assistance
- HRV stress test
- Vo2 max
- Functional power threshold
Why do you want a watch - will you be using it for activities other than riding? If riding, will it mostly be MTB?
I just upgraded a Fenix 5X to 6. I have a few Garmin Edge units. Garmin watch handles bike activities pretty well, but having to look at a wrist watch when riding seems sub-optimal. Wrapping it around your bars, likewise. Just use an Edge when riding. But to answer your queries:
SPO2 - Now I have a Fenix 6, I know my SPO2. Though I already knew it because I bought a $20 Pulse Ox sensor froma ebay a while back. I don't continuously track it on the Fenix 6 because doing so chews much power - loses 35% of battery life. And unless I'm getting sick, I don't expect it to change. Doesn't seem overly useful - or is there more that I just don't know about it?
Incident detection - Never used it on the watch. On an Edge while MTBing it's crap: Race up to a gate or something, jam on the brakes to stop and push gate open, just as I restart riding "Incident detected"! Then I have to press the screen with a gloved finger on my Edge 820 which is just crap at sensing touch with anything less than ideal conditions. Then I fail to cancel it in time and it sends a message to the better half, who ignores it. Oh, and it's only turned on because I absent mindedly looked at the page for it in Garmin Connect app - which turns it on. To turn it off I need to find the setting ON THE EDGE UNIT buried layers deep in the settings ... (located in a disused toilet in a basement, inside a locked filing cabinet with a sign "beware of the leopard.") YMMV.
HRV stress test - seems like a reasonable tool to assess recovery/any illness, but needs a HR chest strap (get one in a bundle with an Edge.) Also, Wrist PPG HR on the Fenix 5X is average - easily corrupted on sunny days when running, paddling etc moves your arm around. Fenis 6 seems a bit better. Chest strap ECG HR is much more reliable.
VO2 Max - running VO2 Max is calculated on-watch with just HR and GPS/accelerometers. Or you could just see how many laps of the local oval you can do in 12 minutes (Cooper Test.) Bike riding VO2Max requires a power meter: Yay, more toys to buy: Stages XT left-hand crankarm perhaps the cheapest MTB PM that isn't crap? Assuming your bike has clearance.
FTP - as per VO2 Max. These are pretty meaningful indicators, but they don't tell you much more than, say, regularly recording times on your favourite climb whilst keeping your HR at similar values.
So, if you're mostly MTBing, get an Edge. If you run a bit as well, I would get the most expensive Edge you can afford, after putting aside some $ for the cheapest watch you can cope with - Vivactive 3 or Instinct have recently been offered for just over $200. You can of course run with the Edge in your pocket, too - especially if you use the "Edge Plus" Connect IQ app. If you swim, and maybe paddle, XC Ski, etcetc then consider a Fenix (or Forerunner.)
Or go Wahoo instead.. Whatever. Just start reading dcrainmaker for real reviews of all of them. He does tend to gloss over Garmin's shortcomings tho: When I bought my Edge 820 I thought it would be so much better than my old Edge Touring, faster, way more features, capacitive touchscreen a step up on resistive, etc. No mention in the dcr reviews of how poor the touch was, nor how often they crashed. Took a couple of years to get reliable. No complaints about the 1030 tho, apart from price, and that it isn't suited to MTB cause it's large and a stack will break it and then when you repair it yourself you'll cause other issues. The 130 is tiny, light, cheap given the features (nearly everything except maps and wifi) and the monochrome screen makes it super readable.
TL;DR - bear already said get an Edge 530/830.
PS: If you think a $500 Edge / Fenix / other sporty Garmin with maps will also give you excellent re-routable directions like, say, a $200 Garmin car navigator, you'd be very, very wrong.