We’ve been exploring a thankless solution, but the company that quited us said it isn’t a good idea in our area because the ground freezes in the winter. We don’t live in a super cold area, but it does snow a few times a year and it can get into the single digits of degrees Fahrenheit.
The tankless heater should have a map in the specs divided into zones by average water temperature. There’ll then be a table that references the zones and what size you need for what you expect to heat at the same time.