I buy the big bags of Miracle-Gro potting mix at HD - 3 bags or more at a time. It has slow release and other micro-nutrients in it (so they claim). There will be some rough material in there - sticks, once in a while construction gravel but overall, I've had good luck with it and easy to find. Slow release for young plants or water soluble (weak weekly addition). For more mature begonias I use the granular plant fertilizer with no issues. Some folks mix their "secret formula" which I long ago practiced but it got to be expensive, messy, and time consuming. Been there, done that.

Don't overthink this. Light, water, good potting mix, and a little fertilizer go a long way. Let the plant tell you how to proceed. In other words, there is no "one size fits all".

Thank you very much for your response.

I have what folks think is a boliviensis hybrid which volunteered in my old old garden. The internet said it likes "sharp drainage". I put it in african violet potting mix I had - lots of peat moss and perlite. What do you think?

Jackie