32 Bit Stroke Effect? I only see 8bit

You will have no color depth problems if you use a shape layer with Trim Paths applied instead of the Generate>Stroke effect. It renders a little faster, is more flexible, and can be used instead of Stroke with no problems. You may need to use a track matte, or set matte, but that's the way I would go.  I can't remember the last time I used Effects>Generate>Stroke for anything.

I looked at your project and I still would not apply Stroke to that image. I would animate a rectangle shape layer size and use that as a track matte, put a copy of the rectangle above the track matte with the size tied to the layer below that only had a stroke applied. If you tie the Position of the Shape layers to the position of the image you can avoid the goofy line thickness change as the scale of the image is animated, have a lot better control over the stroke, and make the whole animation look better. It's a little more work but the end result will be much better and you won't have to put the HDR Compander above and below the stroke effect to keep the layer 32 bit, and YES if you don't use the Effects>Utility> HDR Compander above and below the Stroke effect, that whole layer becomes 8-bit color. 

Here's what the comp would look like. It took me less than 5 minutes. Note the expressions tied to scale so the stroke width does not change:

Here's a comp so you can see what I have done. Just replace the footage with your own: https://www.dropbox.com/s/vdwdjl5fuad88a5/stroke%20project.aep?dl=0
(note: if your browser adds a .txt extension to the AEP just delete it)

Hey Ben 

a shape Layer can do this 

and 3d Stroke from Red Giant. 

https://vimeo.com/388572524

hope this helps 

It's okay to use 8-bit effects in a 32-bit comp. The only serious problem is if you're using it in an effect stack where it would clip otherwise overflowing data.