@usatan - Each fabricator is on an independent conveyor line. That's the main feature of the build. Although it has a bigger silo capacity than the small harvester, so will still take a little longer to finish bricking up than that one, if it's full.

The middle drills should mine rocks fine under most circumstances (the side drills are more for trees). Did you check out the video? E.g. here: https://youtu.be/56nw-7tyxvk?t=1327

Only 2 fabricators and single line conveyor are too few for harvester.
Too height vehicle height doesn't allow drills to reach surface rocks.
It is too slowly for to works well.

It it really a harvester???
(It's looks bit cool, it might be a "family model".)