I'd look at the cottage pinks - Dianthus deltoides or gratianopolitanus. Both need very good drainage.

I would urge you to consider verbena "Sissinghurst". I gardened in Zone 6 for 30+ years and this was planted and thrived on a slope near the front door. It not only looked good year round, but the long strands would root along their length and did a wonderful job of holding the soil in place during heavy rainfall.

I bought my original plant from Bluestone, so they've been selling it for a lot of years and for good reason - it's a winner.

Here is a link that might be useful: Verbena