I do see where you're coming from and also delved in Manual J calcs. Aerospace engineer and work in utility scale power production so I've had a fair share of energy modeling done. You're also right that the basement location will start getting colder and pull heat from the floor above to keep heating itself.

If we do end up using rockwool comfortbatt and insulating in the joist cavity, we might avoid that energy pull from the floors.

My main reason is to eliminate a propane consuming thankless heater provided by the builder. The Navien 240A2 has an estimate of 450ish $/year assuming 2.4$/gal of propane. The electric bill on the HPWH is est to be at 100-150$/year.

If we live in this home for 5+years then that is a lot of savings. Now that you mention the added bonus of placing it in a warmer location, I'm tempted to place it in the garage but the install charges would be high and the winter will make that garage way more colder than the basement.

This is a good start for anyone more interested in the deep research on the topic

https://www.energyvanguard.com/blog/will-a-heat-pump-water-heater-freeze-your-basement/