All these replies are getting confusing!

Do you think the hot at the taps (not shower) is significantly cooler
than before?

If not then surely the shower is the issue? two different thermostats
giving same tank temp suggests tank temp itself is not the problem.
65degrees is steaming hot! if you are getting 56 at taps then my
opinion is shower/pipe run is the problem.

maybe a drop in mains pressure is preventing venturi from operating as
well. was the immersion replacement the only work done?

60-65 is the recommended temp for tank, you shouldn't need it higher
unless heat is being emitted over long pipe run. Lag those pipes!