John, 99% of my Semps are grown in pots, although you and I are in different growing zones.
They are in full sun, but on my deck, which is semi shaded by the house part of the day.
The sun is very hot and bright here in NC. And it seems the sun is shining almost every day (unlike New England, where I am from).
I agree with Chris, your pots will be full and overflowing in a year or so, I bet...or you can plant close together to get that look you mentioned liking in your post.
I give mine liquid fertilizer every now and then, but once they get well established I stop that, as I would need to divide too often and get more Pots!
Semps are such easy, undemanding little plants, it's hard not to love them.
I look forward to seeing your progress with your beautiful pots!