Answer: How much to fertilize and with what really depends on the fertility of the existing soil. I would suggest you work with your local county extension to run some basic soil tests and then based on the test results, work up a plan.
Many plants do not need much fertilization and too much is worse than too little, so it pays to be cautious. Usually a general purpose granular or slow release fertilizer such as 10-10-10 is fine, read and follow the label instructions. You could also use an annual top dressing of a good quality compost. Then maintain a layer of organic mulch two to three inches deep on the soil surface. This will help feed the soil slowly over time as it breaks down.
Since top soil is not regulated, testing may help you identify any issues related to drainage or pH as well as fertility. So that is another reason to run the tests before you start fertilizing.
Finally, I am not sure what you are calling a mulch bed top dressing, however if you have applied six inches of mulch over top of the soil then that is probably too deep, especially for the Japanese maple. A layer two to three inches deep is ample. The reason for this is that roots need air in their soil in addition to water. If the mulch is too deep the roots will begin to grow upward looking for air. If the roots grow through the mulch instead of in soil they will not be well insulated in the winter, and since the mulch dries out faster than soil they will also be subject to drying out.
Good luck with your landscaping, sounds like you have done a huge job!
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