If you are using a good organic soil mix that is substantially compost and/or composted bark, no fertilizer should be necessary. If you are using an inorganic substrate, any balanced fertilizer will do. I like Osmocote-Plus which includes essential minor elements. I use it at the rate of one-quarter to one-half teaspoon per pot-gallon of substrate. One dose is good for about 6 months. Some growers include a similar time release fertilizer at comparable rates in their organic soil mixes (which are substantially bark chips).
I like osmocote too, because here in France we're stuck for options anyway. I use the Hydrangea version. Note my substrate is mostly inorganic. I've heard good things about Foxfarm in the US, and I know @JT1 has some stuff he likes. Indeed, most in the nursery trade, for better or worse, use some kind of osmocote, as they're facing a necessity to grow plants out quickly. Of course for maples this is not always the best solution and often leads to problems once they get out of the controlled environment of the growing shed.
The best fertilizer for Japanese maples in container, bonsai and maples in your landscape, and all landscape plants is PHC Roots: https://www.amazon.com/Lebanon-Healthy-Start-3-4-3-25lb/dp/B00ECLR08I You can use it now. It will not burn, cause salts to build up in the soil, and has rhizobacteria which is present in Forest soils, but absent in our container soils and landscape soil. It benefits maples in many ways and promotes sustainabile growth and better / longer lasting spring and fall colors. Does not use up plant sugars like synthetic fertilizer or encouraged pest, disease, and bacterial infection. You will notice the benefits in the first season, but really notice significant changes after two seasons. We have a large collection of Japanese maples, conifers, flowering shrubs, and dwarf perennials; all respond extremely well and better than any other fertilizers including Osmocote. Now things are extremely healthy, full, and free of pests and disease; much more tolerant of extremes like cold, heat, and drought. I do not benefit in any way by my recommendation or have any connection to the seller or manufacturer. I found this fertilizer after doing years of research and trying many fertilizers including some that were $100+ for 25lb bag. Stay away from cheap fertilizer and also the most expensive does not make it the best either.
