First of all let me welcome you to the QB from SE Michigan! Glad to have you here. I have not dyed a lot of fabric but had 3 yards ready to go right now that just finished their "pot ash" bath and are ready for dying. I have not worked before with Dharma dyes. I suppose I am one for lesser cost effective dyes. I use RIT dyes. Something simple. I would maybe prep your tshirts a bit differently. Pre-wash in detergent without fragrance, and then the bath in pot ash before starting the actual dying process. After that leave your dyed tshirts in the hot bats for as long as possible keeping the water hot for as long as possible. I keep my tea pot ready with the hot water. I use the bathtub and for the fabric bats I use wall paper trays for each color dye I want to use. I have not tie dyed any fabrics since my hippy days back in the late 60's The yardage I did a few years ago came out just the way I wanted it to looking like a nice blended fabric. What I am preparing now will be with a bit of design, more batik-ish than others I have done. The fabrics I am doing are a guild project that COVID has kept us from getting finished together to complete this project.
Don't give up. You are so on the right track. Again welcome to the QB. Looking forward to seeing a lot of you here.