If I'm using cap strawberry sweet I add it about 3 days before the mix is fully steeped , because it fades.

Adding a few drops of the fruit part of the recipe after a long steep helps to brighten it up. It doesn't take much.

You can leave out the concentrates that fade and add them right when you're ready to vape the juice. For example you could mix the pie/custard base of a lemon pie and allow that to age and get richer, then add the lemon afterwards. Or you could mix large batches of a custard base and let it age, then transfer some of it in a smaller bottle, add whatever fruit you feel like on that day and be good to go. These are just examples but yeah it's something that can be worth doing.

Agree with previous comment but will add that I’ve added flavor after steeping a couple times. I would try to up the percentage of a specific flavor and add a few drops into an aged mix. The added flavor tended to overwhelm the mix rather than just brighten the specific flavor note I added to. I haven’t done it since.

Well adding something after steeping kind of defeats the purpose. Have you tracked it down to a certain flavor?
Edit: reread your post. Maybe what you need is a stone. I think that's what you're after.