Serious gardener here. For your flowers and veggies, find a farm supply store, google or read the boxes to find out what's in the organic, slow release fertilizers. You can then buy these components in bulk individually and mix your own fertilizer.
Miracle Grow, etc are just distilled compounds, but the organic options will usually tell you what components they're using (lime, canola meal, etc.) So For instance, I make up a complete fertilizer for my veggies from alfalfa meal, lime, rock phosphate, bone meal, and kelp meal. Slow release, works awesome. Do some digging online for recipes suitable for your region and soils.