The best answer is a conventional GAS water heater (burner under the metal tank).
Don’t pay “extra” for a longer warranty, its just insurance, the water heater is no different.
When it craps out, buy a new one.
No draining/flushing or suffering with low water pressure.
Also “brand” (in conventional water heaters) is meaningless! Get the best deal you can. “What’s on SALE?” buy it!
Having personally sold/replaced/installed HUNDREDS of water heaters, I know what I am talking about.
I have replaced many Monel tanks in my day. They didn’t last forever, but I would tell my customers who asked about replacing their very old looking Ruud Monel tanks to wait until it leaked because it was the best water heater ever made. No telling just how long it might last.
140 degrees ONLY! Any lower grows bacteria.
On demand water heaters SUCK!
Indirect fired water heaters (a separate water tank heated by your boiler by a coil) are great. Especially when coupled with a high efficiency “gas” boiler.
NOT a coil IN THE BOILER ITSELF! (VERY BAD IDEA)
So, you know about the Sacrificial Anode?