I am not getting many large particles from sifting, so am looking for an alternative material for the initial drainage/aeration layer in the bottom of the pot. How about using pea gravel sifted to 1/4 - 3/8 inch?
It should work but be a little heavier. Round pea gravel will not pack as tightly as the irregular particles of pumice or lava, but more like akadama. I know that Ryan recommends using the same mix as the main soil mix but in a larger size. However, I normally just use pumice in the larger size and it works well.
The reason for using the same material would be not to break the water column. Different layers could effect drainage by different absorption or capillary function.
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I agree that using the same material will lead to a more continuous water column. But one of the reasons to use an aeration layer is to provide a break in the water column due to the particle size difference so air can come into the bottom of the pot. With the thin (1 - 1.5 particle thick) aeration layers we use and the fact that some of the upper soil will fill the inter particle voids there will be a minor break in the water column even with a very different layer like pea gravel.
I do the same as you for most trees especially in taller pots a maple in a shallow pot will however at least get akadama mixed in.
i assume you know the information in the following article but it is something everyone growing bonsai needs to understand. @Chibon