Just an incomplete answer: The small stuff acts like a filter in the low end towards
the grate. A typical Imbert property, when using a fine stationary sieve instead of
a proper grate.
Still, you use a grate with about 1/2" slots! This tells, that the reduction volume is
too big (passage area big and draft velocity low) for the small fuel size and driving style.
After the restriction, one has to keep up the gas velocity to avoid the "filter effect".
Preferably slightly increase it!
This is of course against the talk about residence time, but what good is there with
residence time, if the char is clogged by ash? Ash-clogged char cannot act chemically,
as the active surface is "insulated" by ash!
I think you need an active sieving + self cleaning angle-bar grate,
operated manually at about every time you bunker, if you insist to have a "lazy"
reduction volume after the restriction.
The "lazy" wide reduction volume will then work with big size fuel too... with heat.