I’ve tried a few different mixes over the last few years, and honestly nothing has worked great compared to planting them in the ground. Some percentage of potted seedlings show root stress every winter no matter what they are planted in. I think our cool temperatures, even in the greenhouse, make them prone to root issues. In the ground, they always look very happy other than freeze damage.
What I’ve mostly done is order a truckload of “garden mix” from a local bulk soil company, and I mix that with coarse perlite to improve drainage. However, I haven’t found a local source of bulk perlite that’s at all affordable, so this year I decided to try adding more sand to the mix instead. It results in a mix that is probably too dense, but it’s a LOT cheaper, so if it ends up working ok then I’ll just do that going forward.
I posted about the garden mix here:
The only modification is I’m adding about 20% more sand for the new seeds this winter, instead of adding perlite.