More dose doesn’t necessarily reduce bitterness. Bitterness could also come from more material get extracted. The real over-extraction signal is bitter in long lasting aftertaste. If reducing dose leads to balance then this is the case.
If ground suddenly lose pressure for no reason, it is probably because CO2 is leaking too much since beans are not consumed in time, just grind half level finer (Niche Duo) and it should be fine again.
It is important to discard some ground right after changing the grind settings, otherwise different sizes of ground will mix together and make channeling happen more easily.
Over-weighted dose could also lead to uneven extraction because the puck expands all the way to the group head, leaving no room for spreading water evenly.
Sweetness is an attribute of a certain bean. But it may need higher temperature.
To having more acidity:
When doing espresso with ice and fresh milk without frothing, bitter will become obvious, extraction needs to be lower while keeping high concentration.
https://www.youtube.com/watch?v=4dRYw8yl_dg
https://www.youtube.com/watch?v=1qyFDbkRrOA