Sizing zener and resistor for solenoid flyback
220V through 165ohms is about 1.3A.  The rise time for 0.865H and 165 ohms is about 7msec, so at 50Hz on/off the current will pretty-much reach the steady state.  So every time you turn it on, you're putting 1/2 i^2 L = 0.7J into it, and when you turn it off you need to pull 0.7J out:  That's about 35W that your flyback will be dissipating.

The point about reduced pump pressure with just a diode may be because the pump wants to dump the energy fast (into a zener or resistor) so it can return to its previous position in time for the next stroke. Just a flyback diode lets the current flow mostly unimpeded, so the solenoid stays pulled for longer.

If the pump really does want a nice, smooth sinusoidal drive, you could try to find a 120uF high-voltage AC capacitor and see if you can create a 50Hz parallel resonant circuit.  Two warnings:  Check that value, I am working on my phone and it might be wrong.  And second: Make sure you get a high voltage AC capacitor, i.e. for a motor start.