My experience is with Promix bought in bales, that seems to work fine. What are you trying to grow? Maybe you will want to just buy the components in bulk and mix up your own. You can grow a lot of different kinds of plants in bark mixed with sand, with slow release fertilizer added. Use a higher proportion of sand for those types wanted more drainage or aeration than usual.

I'm using the winter sowing method for all types of seed starting even vegetables in re-purposed white plastic gallon milk or water containers.

Promix looks like it is peat and perlite or similar, probably fine for your purposes. I haven't actually had occasion to read the label, see what's in it.

I use mushroom compost, $27/yd., mixed with a little vermiculite, perlite, and coir or peat moss. Vermiculite on the top. Works great.