Your stove has 5” spigot so brush would have to be very soft to go through it and expand into the 6” section.Difficult. 900 from stove to closure plate, 750 from closure plate to ceiling (within breast).
How do chimney sweeps do this? Do they have equipment to get into the flue from within the stove? Hmm.
The other way to sweep would be a power whip which it easy to get through but some stoves have difficult to remove baffle plates.
litl