You can prune it as a shrub or as a small tree- it's entirely up to you.

I have some that I prune up as a tree then lop the top for a neat canopy. Others, I wait until after bloom and I cut them way back so they continue to grow into a shrub form, otherwise they get too leggy at the bottom.

Don't be afraid to hack it back. It will come back fine in no time.

Ok, got a pic this afternoon. It's between our fire pit area and swing set, on a little berm. The chartreuse next to it is an ash who also moved in last year. So eventually it will be an understory planting. Should I make it a tree or bush? Opinions please!

My own personal opinion is to let it grow however it wants, then as years pass prune to aesthetically enhance that look. The light might determine which way it leans, for instance, so work with that rather than against it. If it gets top heavy of course I'd take something off the heavy side but let it determine it's own general form.
YMMV- if you strongly desire a tree form start pruning earlier to direct the growth. How we shape plants is highly subjective- look at the branch structure in the winter and make a plan for spring pruning- I stare at things for a very long time before I prune.
I'd just let it be for a while and get to work on that fencing :)

Laughing out loud about the fence comment. Gotta love it when people critique things other than the subject (being facetious).

If you are letting the ash grow as a tree, I'd cut the sand cherry down as a shrub so they aren't competing for space.

Personally, I think the sandcherries growing up into a tree form look better en masse than just a singular plant.

You think? Yes, the fence is obviously at the top of the never ending yard list. Pruning is free though. :)