thanks to all the neighbors who shared their experiences and recommendations!
I ended up going with HomeServe since I have that insurance. The plumbers they sent were D.R. Sevacko. I had no choice. The insurance would only replace my gas water heater with the same. I paid only $195 for the expansion tank that is now code and that I didn't have before.
But I feel much smarter about it all. In the process I also met Golan Barak, a plumber I liked and would have hired to do the tankless, but I couldn't hire whoever I wanted.