Sites like Senmanga are terrible for their raw scans' quality. Where does everyone get their manga raws?
The ones uploaded on Nyaa are occasionally decent but are mostly hit-or-miss.
Nyaa, jcafe. Plus some random sites off google.
For volume scans you're stuck with the hit or miss quality since no one uploads better versions of anything if someone already uploaded a volume, while magazine scans are usually extra bad which is what I see mostly on sen.