I would suggest accepting the extra 20 litres an hour and just go for a twin version of what you have already!
Find a decent Seneca III. Better than the II in a few ways - extra 20 hp on take-off or EFATO, better cockpit layout, better weights (I think the MZFW of the II is limiting, although I only have a few hours on that so don't quote me).
Basically Piper never improved the Seneca from the III, just made it heavier with gimics.
The Seneca will be lovely and familiar to you. It will be cheap to buy (probably under US$100k) which makes up for the fuel burn over the Diamond or Tecnam, although not as cheap as a Comanche. However it won't be as old as the Comanche either, and it is larger. The cabin will of course be very familiar to you, as I understand will the handling.
The Seneca III is a great aircraft.