I have a bunch of plant food sticks described as "compound organic and inorganic NPK 12-6-9 fertilizer with micronutrients for ornamental plants". I wonder if this is suitable for a Ficus (Ginseng?) bonsai?
I do not recommend solid fertilizer sticks for small plants. When the sticks get wet they release enough fertilizer to burn the delicate hair roots that absorb fertilizer. As the soil dries out the roots grow towards the fertilizer and get burned again when the soil is watered again.
This guide recommends putting spikes at the perimeter of the pot for this reason. However a bonsai is in such a small pot you don't have a lot of room to work with.
With larger plants outdoors they can cope with this but an indoor bonsai is not as robust.
The formulation is not appropriate for a bonsai as it will promote leafy growth. For bonsai you want to direct the growth to achieve the look you want. If the plant is popping out leaves due to the higher nitrogen levels then you will have more to prune.
I recommend a 20-20-20 ratio applied three to four times a year in a water soluble solution as this will promote an even ratio of foliage and roots. Even better get an organic fertilizer that you can apply more often but is more dilute so you avoid increasing the available nitrogen by a large amount. You can avoid the possible danger of root burn and provide many other trace minerals which may not be present in a commercial fertilizer.
I've taken a couple of Bonsai classes from Professionals and they've recommended you either use a bonsai fertilizer or if you use a regular fertilizer, then only use half or a quarter of the strength. They recommend fertilizing more frequently at a weaker concentration as opposed to full strength once or twice a year.