I don’t see any brood disease.
That dark area between the brood & honey arc looks like stored pollen to me. You can see very dark pollen in some cells that have been covered in honey beneath the honey caps.
If that’s the most brood the bees are producing on any frame, that must be the limit of that colony. Unless the colony has nowhere to move honey to. Like in the roof, for example. It would be interesting to see what another colony in your area is doing.
It’s always good to have more than one hive so that you can do comparisons. Alternatively have a friend nearby that has beehives.
Edit: To answer your last question: It probably is a low amount of brood. However it should be taken into context of what’s happening in your area. If it’s low compared to other colonies in your area, the only solution I can think of would be to re-queen the colony.
Upon closer examination, it almost looks like those dark cells are honey & not pollen. The bees may have found some really dark honey somewhere. If so, that would explain the low amount of brood. The bees are not finding the pollen needed in order for them to raise lots of brood.
There’s no substitute for physically looking at the frame up close.