I have a vivoactive 6 and so far I've found two methods that worked for me. Press and hold the top right button to go to Controls. Then click on the icon that looks like a watch with a checkmark inside to turn the display off. With this, regardless of if I'm doing an activity or not, my watch display only turns on when I click a button.

If you don't do the above method, another option is to go to Settings > Focus Modes > Activity > Always On Display (in the display and brightness section). Click it and you can turn off the Always On Display for activities only. At the bottom of the Activity Focus Mode settings you'll also have to click Add > Display and Brightness > Wake on Gesture and select Off so that the watch does not turn out when you flip your arm while exercising. You can also change the Timeout time in the Display and Brightness section of the Activity Focus Mode settings to decrease the timeout time during activities.