I start in pots only things that absolutely have to be started indoors - tomatoes and peppers. I eventually settled on pots called sili-seedlings which are silicone and washable after some attempts at soil blocking (which I plan to try again in the future, still think soil blocks are very much a good idea).
Buying specific seedling mix is expensive and not truly sustainable. However, our soil is pretty heavy clay and our compost is somewhat on the heavy side too. I don't buy/bring in any external inputs or fertilizers aside from small amounts of
sea crop and
effective microorganisms. The other exception is I did buy a bag of locally made peat-free potting soil last year. It is light enough for seedlings. I am okay making this exception as the bag will last me a while.
If this product was no longer available, I would try to somehow lighten up our garden soil with soil from other parts of our property.
I will note that the seeds we save germinate much better in our soil rather than store bought. It also means that you have extra seed on hand if your germination attempts fail