10mg probably lowest effective dose, but per article, proper dosing requires blood work as it does for all other hormones. Again, studies tended to use much higher doses than I'd recommend, at least to start.
Compounding Pharmacies make 2.5-5mg DHEA Suppositories.
DHEA Vaginal Suppositories 30ct
My wife and I went to this place long ago and bought these. They will make higher doses with script. 2.7mg you can get otc. I don't think they did much but if your lady don't have desire for sex she probably will have no desire to use them. Like a person with no appetite to eat is not going to eat.
A different route of admin, so a different topic. Not surprised 2.5 mg via that admin route didn't have any effects. What were DHEA and T levels pre and post use?