Heartrate so inaccurate that my watch is basically useless for training

This definitely sounds like a problem specific to that watch or your heartbeat pattern. I haven't seen a comment like this before and I was scanning the forums before I bought because of the issue many runners are seeing. I'd get it replaced or work with them as you may be a one off. Halving or doubling of HR was a problem with WHR sensors like 4-5 years ago, but tripling, quadrupling, is a whole other issue.

Frankly, Garmin should add you to their deep metrics program, giving you a watch with software set to collect software and hardware metrics, not just a incomplete .fit file. A fit file isn't going to give them nearly enough information to get to the bottom of what is going on. You need to have the software and hardware in a debugging mode so it spits out deep metrics information for engineers to look at.

I have been wearing an Apple Watch since the first version and have never had an issue with HR accuracy. My only issues with it is the update of the HR on the watch display. It updates at random and the data can be minutes or hours old. I can get around that by running a workout throughout the day but there is no way to end the workout without saving it and it messes up my Strava workouts with non workout data. But I never expected the Apple Watch to be a 'Sports' watch. As I mentioned I tried the Fenix 7 pro and the FR 965 and got similar inaccuracy. When I tested the Fenix 7 pro I wore 4 devices during the virtual ride. The Fenix 7 pro using optical, my Apple Watch 8, my Polar optical arm band connected to Zwift, and my 920XT connected to my chest strap. During steady state effort the Apple Watch, Polar arm strap and 920XT w/chest strap read within 1 BPM while the Fenix was 10 or more BMP off. When I was slowly changing exertion the 3 other devices incrementally changed readings while being in sync on the value while the Fenix often just sat there without changing the value.

I know for a fact it is not just me or my particular 965 device. I see so many other HR accuracy complaints regarding Garmin watches on this and other forums. I also watched a video review of 40 different optical HR devices done by a scientist with a scientific grade HR instrument as the reference. The Apple Watch came within a couple percent from the reference device for accuracy while the Garmin FR965 faired in the low to middle of all 40 devices where the reviewer specifically commented about a lot of the inaccuracies he found in the Garmin FR 965 HR and sleep tracking. For people who have no other device (especially a chest strap) to compare with they never realize how inaccurate the Garmin optical devices are.

I don't know how you can make the claims you have about comments from others where as I write this in the side bar I see this in my screenshot. I have seen so many complaints from so many users that I cannot understand how Garmin can still sell these watches and you can make the claim regarding complaints that you have. BTW - check the title of this thread!

I know Garmin likes to point out all of the inaccuracies with optical devices under varying conditions as an excuse but I will fall back on my original statement "I have Never - since my first Apple Watch Series 1 to my current Series 8, have I ever had any HR inaccuracies with them (other than the lag I would expect from an optical device). Every time I saw an unusually high reading on my Apple Watch (and not 200s but low100s when I was at rest) an ECG confirmed I was in Afib. I would bet my life on the Apple Watch but absolutely NOT on a Garmin optical HR device.

For me my HR is always low until the mile marker or performance index notifications. I'll be on a 30min 139bpm baseline run. Up until either of those notifications or will stick below 120 even though I'm kinda hoofing it to see if it will rise beyond that. Maybe an 8min pace. Then one of those notifications comes in and suddenly I'm at 159. Then for the remainder of the run it's pretty accurate... I guess? It seems accurate and it's consistent at least.

More data.

What are we looking at? What were you doing vs the time?

It really doesn't matter.  The two massive HR jumps are unacceptable and are excellent examples of the issues at hand in both increasing and decreasing HR. 

It does matter, HR jumps happen when you're doing intervals and such. It looks like you stopped, HR went down, then started again and it shot up...normal after a warmup, though it looks like you pace was a lot slower so HR should not have spiked. Same with the decrease spike, you only stopped for 15 seconds maybe yet a massive drop in HR and then it didn't recover to where it was previously. That's what you need to explain, not just some silly graph, here is the data...you need to do the work explaining the situation or provide much more detailed data.

I'm not going to fight with you. 

There's plenty of information for Garmin, not you, to take on board.  

It's not a fight, you clearly don't understand how data is used or how engineers work to reverse engineer problems. They will ignore your graph, as I'm sure it's useless to them. What might be helpful is the fit file, detailed "just the facts" context, etc...

If you don’t want critical comments about what you post then just contact Garmin instead of community forums.