Apple watch 8 charging during activity

No. If you are using the watch to track activity is would have to be strapped to your wrist, with the sensors touching your skin. There would be no way to charge the watch in such a situation. It needs to be charged before the activity.

You failed to say there would be breaks in the activity. I know you can pause an activity, but putting it on its charger, may start a new activity. I can't confirm that. You'll have to try and see what happens.