Buy a bag of seed starting mix. I like Pro-Mix but it comes in big bags. Look for something that has peat and perlite.

I use Pro-Mix BX.

Two stages to growing seedlings - germination and then growing on. Under some conditions different mixes are used for the 2 stages. So when you title your post with "starting seedlings" it isn't clear which stage you are talking about.

You mention "a seed starting soil". Which brand? Seed starting soil is what many use with no problems so your problems may not be the stuff you used at all. The peat pots or any of several other issues may be the cause.

Orchid mix is a good growing mix but the bark fines it contains can be problematic for germination and it can be expensive.

ProMix is the most frequently recommended germination and growing mix. I use it too. But it can be difficult to find in some locations. The next best IMO is Metro Mix 360. Not that I am recommending it but many use nothing more than green bag Miracle Grow for both germination and growing on.

Dave