You would want to fuse every circuit connected to your source, whilst all the sub-circuits from the ATO fuses would be fused, what about the cable from the AC200 to the fuse box? If that develops a short, no fuse = tripped internal AC200 fuse*.
Typically you would take the expected load and multiply by 25% get your fuse value but, as your Bluetti DC output is limited to 25A, a 25A fuse makes more sense. An inline ATO fuse would be fine but if I were doing this I would fit a bolt-down midi fuse like the
littlefuse 498 or flex series. That's just a personal choice, either would be fine.
Note: It seems a shame that this Bluesea fuse box doesn't have an incoming fuse, they even suggest fitting one in the schematics in Amazon's product description.
Actually, thinking a bit more on this, I would want the external fuse to blow before the internal fuse does, so maybe a quick-blow fuse would be better for this application? You might want to research this yourself. I recommend checking out littlefuse and blue sea.
*I am assuming that the AC200 has an internal fuse and that it is not user-serviceable. If it has some kind of re-settable breaker, I probably wouldn't bother with the external fuse. Sorry, I am not too familiar with the Ac200 specs.