No need to do any of that.
The perfect companion to an on demand water heater is a Shower Miser. https://www.showermiser.com/rv/
We installed one years ago in our RV. It saves us about 30 gallons a week while boondocking, while also eliminating any cold water bursts when taking a navy shower or warming the water initially. We love it!
Not sure what you are experiencing with your unit in regards to 'high flow necessary' for an on demand. Ours is one of the first generation, and if you turn on the hot water above a trickle, the unit fires. Perhaps your unit needs adjusting or you are still going through the learning curve?
We always turn on the hot water only to full open, and then adjust from there. Newer units have several different methods to set temperature.
Scott and Liz - Southern NM
2012 Wildcat Sterling 32RL - w/level up (best option ever)
2007 Chevy 2500HD 4dr short bed Duramax w/allison
Reese Fifth Airborne air ride king pin coupler with Sidewinder