Have you ever tried to contact support? Maybe there is something defective.
In fact if you say " to use “best” accuracy which does not even exist anymore in the menu" maybe there’s something wrong.
Did you check power saving menu ?
Do you have the latest firmware ?
I use GPS only and I never face drift that can let me say that the watch is unusable … HR(if you meant OHR) it’s a different topic. It could simply not work for you.
@richard-j-adams Well, incredible as it sounds, Suunto does not look like is willing to invest in proper structured sessions like the one you described, even though previous models had that available. Right now you can program that “simple” session in some other brands, but not with Suunto.
Don’t even think about asymmetrical intervals or ladders…
I would say the nearest you could set is a 20x50m with 30” rec time interval training. You would be able to do warming before the exercise (you would press start the exercise once you are warm and ready), and after you finish you would have free run, but you would need to know how long you have been running after the interval training and that’s usually hard to do without reading the watch face.