Could be it's the watch, it's a Garmin Vivoactive 6.

I've ignored one-off high readings that only last for like a second spike, I'm going by specific heart rates that I peak at and sustain that I can monitor for a few seconds even if I stop moving.

The typical formula is 220-age, but I’ve found that it can definitely miss the mark

Same, and by that metric I'd be dead. 😂 I've hit 190 bpm, and regularly get into the 180s, but standard metric would have my absolute max in the 170s. I can sustain 170 bpm for several minutes without even feeling too stressed.

Finally, it helps to know if there are any underlying cardio respiratory issues that would impact the strength of your heart.

No issues there, good heart health. Occasionally high triglyceride profile (depends heavily on what I eat the day before a lipid panel) and frequent slightly lower than optimal "good" cholesterol on my lipids, but that's about it. Overall cholesterol is low, and my doc gave me a clean bill of health with my heart (literally saw the doctor this morning for my annual checkup, awaiting my blood work later this week).