I am in zone 7a and need some seed starting advice!
I start my seeds indoors, in a small plastic covered greenhouse that I put in a window in my living room. I use grow lights and a heat source. They tend to germinate very well but then seem to rot and die after a few weeks and long before it's time to put them in the ground. I only use heirloom seeds and start cukes, tomatoes (several varieties), eggplant, assorted peppers, broccoli and brussels sprouts. Is it too warm? Too moist? Would appreciate any help!
I wish I had a picture of your set-up as I'm having a hard time picturing an indoor greenhouse and you didn't say what growing medium you use. However, I think the seedlings are too hot. The heat mat is for germination. After that, take seedlings off the mat and out of the greenhouse if, by greenhouse you mean a domed seed-starting kit. Seedlings need good air circulation to prevent damping off and other fungal problems. If your window is south facing and the greenhouse gets direct sunlight, that by itself is too hot when the dome is on. Bright but indirect light is fine while waiting for seeds to germinate. A common problem with seedlings is improper watering and some seed varieties are more susceptible to rot than others. Over-watering is probably more common than under-watering. Both are fatal for seedlings. It is so easy to overwater especially in cooler winter homes. Although it is often recommended, I find that watering from the top until water comes out the bottom drainage holes is too much water for my seedlings. I water from the bottom with a couple tablespoons (let the plant sit in the water for no more than 5 minutes). The exact amount depends on many factors such as house temp, size of container and seedling size. Aim for damp, not wet soil. The following link goes to the entire list of articles on seed starting at this website: https://www.gardeningknowhow.com/search?q=seed%20starting
Check the website of your greenhouse provider and one of your seed companies for more information. I find that seed companies have lots of good online information.