Seed Starting Medium?

Here's the recipe I've used for years:

4 parts coco coir
3 parts perlite, 1/8" size
1 part worm castings or finely sieved compost (I make both at home, and use whichever one is ready)
tablespoon - handful of soil from the vegetable beds (just a little bit relative to the overall volume of the mix you're making)

The coir and perlite make a great texture that both holds water and drains, and is fine enough for small seeds to germinate easily.
The worm castings add a little nutrition and microbial life
The garden soil inoculates the seed mix with the microbiome the seedlings will encounter when they're transplanted, helping them develop their symbiotic relationships early on

I buy a small bag of pro-mix or similar myco potting mix or seed starter if I can find it, the next voyage to a town big enough to have a hardware store. Beggars can't be choosers, but this year I have some myco lightweight soil that has been waiting under the toilet paper all winter!

I start prescious little indoors because space is at a premium -- an incandescent automotive trouble light hanging in the bath tub with seedlings in trays in recycled milk cartons and other recycling containers -- and if I wanted to get my own soil I would have to prepare it in fall and have it accessible for bringing inside to defrost so it would need to be under cover because there are several feet of snow. Edit: Gatineau mountains Quebec Canada

Right now the only thing I am sprouting is buckwheat for sprouts and grapefruit seeds for a 3 season supply of new leaves to nibble on and make tea.

In March I will start a few Jimmy Nardello sweet peppers from my stock of saved seeds sitting in the freezer door. I had hoped to save 3 plants over winter, but no luck. So I just have the peppers in the freezer door.

A few Matt's wild cherry tomatoes to get some early fruit.

Both transplant well, will sprout in any soil, no fuss and don't mind being jammed together and fed a soup of pigeon poop water.

If I want to grow sunflowers again, I must simply locate some of the predator and rodent pigeon cote space to getting the sprouts to about 4-6" before attempting to transplant them anywhere outdoors. Ditto for nasturtium. That means around early May as I need zero frost nights although I am two weeks ahead of 3rd week in May inside the coop.

As I have become more experienced at gardening for produce, I have learned not to buy any commercial seedlings that may bring in contaminated soil or other diseases, grow what can direct seed outdoors and succeed in a harvest by fall, and eat what grows in my climate, depending more so on perennials. I find putting pet cages or other wildlife proof obstructions works well. However sunflower and nasturtium seeds attract the pesky chipmunks and I have discovered I can't reuse soil that has nasturtium seeds that didn't germinate, unless in an inaccessible hanging container, because the chipmunks will destroy a bed of young seedlings in an attempt to root out one nasturtium. That tells me they are super healthy, so I try to grow the red ones that bring in the hummingbirds and again, they will germinate in pretty well anything.

I have a lot of obscure seeds waiting to try seed starting indoors -- fruiting, several Ukrainian varieties of veg, ginseng, and they are going to wait until 2027 because this year, like last year will be too busy for more than basic staple crops like my scallop summer squash that have made it this far through winter in cardboard boxes on the basement slab! (Very happy)
When I start these seeds I will use the commercial mix. I know buying in bags isn't eco friendly but when the quantity is minimal and all soil bags get cleaned, folded and stored for future use, not so bad.

Sorry if this is wordy -- I tend to be over inclusive -- but I hope the information is helpful.