I haven’t started any garden plants inside for years, I do start some in outdoor, unheated cold frames. Some of those are started in pots in my own mix and some directly in the ground. The mix is the crumbly part off the bottom of a rot pile, I don’t call it compost because it has not been properly mixed and turned; it’s just the sifted crumbly stuff from the bottom of a pile of old corn stalks, or bean vines or whatever is handy. I mix it with some straight soil and dry black locust leaves and maybe grass clippings that I collect and save over winter. It isn’t perfectly consistent from year to year, sometimes I go down to the river and get a bit of sand. I amend the ground with that same mix in the starting beds. I don’t use cell packs or things like that, I just pull or lift transplants with a hand trowel.
I have clivia miniata growing in the same pot with the same soil for over ten years and night blooming cereus in the same pot for over twenty years. Christmas cactus have been in their pots for five years. They all go outside in summer and spend the winter with limited light, low temps and little water. Sometimes in spring, when I clean the aquarium and pond filters, I water them a little with the black stuff that comes out.