How to stop the screen from dimming during activity?

Unfortunately, no.

Your vivoactive 5 (and most newer Garmin watches) has an AMOLED display (like Apple Watch and most phones). Garmin auto-dims AMOLED displays to preserve battery life and avoid burn-in.

Older Garmin watches (like vivoactive 4) have a MIP display (kind of like an ereader screen) - this display is reflective, is easily readable outdoors in daylight (no backlight required, so dimming isn't an issue), and is readable at night with the backlight on (the backlight can stay on indefinitely during activities.)

The disadvantage of MIP is that it doesn't look good indoors, it has relatively few colors (only 64) and looks very dull and washed out. People had been complaining about dull Garmin MIP displays for years, so Garmin finally transitioned most of their lines to AMOLED, which is sharper, brighter, more colourful, and looks great indoors.

The disadvantage of AMOLED is that it dims and you have to press a button or turn your wrist to bring it back to full brightness. Most reviewers, bloggers and users say it's no big deal to turn your wrist, and it looks like the market has spoken here since most new Garmin watches are AMOLED now :/.

So basically Garmin has traded in function for form here, and it seems like it's working pretty well for them.

Tbh if you want a recent Garmin watch with a MIP display for running, I would get a Forerunner 255 or 955 while you still can.

While not an incorrect statement this still doesn't explain why the screen dims during activity. Presumably it's the power saving backlight timeout? I noticed it today when the watch invited me to do breathing exercises. The screen dims during the activity so I can't see the prompts anymore. The last time I did this (a few months ago) the screen remained back lit so I could follow the instructions. Something has happened in the last few months? I know I can change the backlight delay but then this will apply all the time so not ideal.

While not an incorrect statement this still doesn't explain why the screen dims during activity.

As I said above:

Garmin auto-dims AMOLED displays to preserve battery life and avoid burn-in. There's no way to keep the display at full brightness during an activity on Garmin AMOLED watches. (Polar has a way to do this, but it burns through battery very quickly)

In contrast, you can change settings on a MIP watch so the backlight stays on forever during an activity (at the cost of battery life ofc), since there's no risk of burn-in.

The auto-dimming behavior of AMOLED watches (and the fact that they need to be at full brightness to be fully visible in sunlight) is why DC Rainmaker has emphasized in his review of Garmin's initial AMOLED offerings that you have to learn to turn your wrist to bring the display to full brightness, but it's no big deal because Apple Watch users are used to it and everyone loves Apple Watch, right?

Presumably it's the power saving backlight timeout? I noticed it today when the watch invited me to do breathing exercises. The screen dims during the activity so I can't see the prompts anymore. The last time I did this (a few months ago) the screen remained back lit so I could follow the instructions. Something has happened in the last few months? I know I can change the backlight delay but then this will apply all the time so not ideal.

I'm not 100% sure what you're asking for here? You want the timeout to be longer only when the watch is displaying instructions? Is that how it worked before? That's probably the kind of thing you'd have to take up with Garmin support.

If you mean that you want different timeouts for general use and activities, that should already be possible, according to the manual (it's possible for my Forerunner 955, too):

www8.garmin.com/.../GUID-24A772E9-96DD-4B65-AC8E-42853AA2C676.html