Also the golden rule of priortization:
For every packet treated to better than average some other packet(s) need to be treated worse. If you try to up-prioritize so many packets that there are insufficient 'victim' packets prioritization will not work well. Or put differently prioritising all traffic is the same as not prioritising at all.
So as a rule of thumb only use prioritisation cautiously and sparingly, always make sure that any prioritisation rule you use has verifiable positive effect on your traffic (be harsh here, if a rule appears sensible, but does not result in measurable improvement, drop that rule).
Good luck!