Always-on display

If I understood these settings are hidden inside the "Focus modes" in the F8 reordering.

fēnix 8 Series Owner's Manual - Customizing the Activity Focus Mode (garmin.com)

Oof I was thinking about getting the AMOLED version and using it for a bike head unit, but this has got me a little concerned.

I was worried about burn in and battery life initially with the screen in for a couple of hours, but it's looking like you may not be able to have it on constantly at all. May be better off using MIPs for these activities that needs the display on constantly.

Keep us up to date with how you go!

Press and hold the Up button [middle button on left side] until it gives you options. Go to Watch Settings. Open Display & Brightness. Select 'Always On Display'. What actually happens to the screen appears to be application dependent. Some apps may reduce brightness. Some may reduce what is shown. Some may shut off the screen.

AOD turned off here, had my first running session today and screen stayed on while brightness got reduced. Really liked it, was perfectly readable in bright sunshine.

nope.

if you cannot use your hand for a gesture (e.g., in rowing) then you are out of luck. 

I'd suggest using a proper sport / outdoor watch instead of this kid's toy 

Yes. Currently, I have a bug on the watch that won't let me change the AOD to on or off activity or general. However, you can change it in the app. This feature has been available since the original EPIX was released two years ago. However, to have the display as "active," you need to wear it. For example, you can't take it off of your wrist and set it down and expect the screen to stay "on"; it will not. If that is what you want, then the MIP display is your better option. 

these are all no answers for the question.

the question was: is it possible to force the display to stay on during an activity?

the answer is: no

it will dim to an extent that it could be impossible to see depending on the ambient light

“on” is not the same as “the same brightness”

and "on" is also not the same as "at least as bright to be seen"