Replacing 2 DV gas WHs with recirc, looking at hybrids and other options

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Basilisk

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My current setup is two 40gal ProLine (NG, direct vent) heaters sitting in a dedicated closet in the garage, connected in series. Heaters are original to the house, with 2004 date code, so it's probably about time to start planning their replacement. The first in series already has a what feels like a bad thermostat, because it would not even kick in unless the temperature is near the highest setting, and for the last couple of years it's been demoted to a tempering tank. Overall everything works reasonably well for its age, and we have not run into any "not enough hot water" issues. My only minor gripe is that the temperature swing at the outlet is about 35 degrees, so water temperature throughout the house is not consistent over time, but I can live with that.
So, what to replace them with?
For an extra challenge, there's an existing recirculation loop with a dedicated return line, and with 4 full baths located very far from each other, oh boy it's a long way to go: Taco 003 circulator takes almost ten minutes to make one complete loop. Turning off recirculation is simply not an option, as I'm not brave enough to tell my family that hot water is going to take at least 5 minutes to arrive every time from now on.

The house has brick siding, so I would like to avoid the massive headache of relocating the vents as much as possible.
Amazingly, AO Smith still makes an otherwise identical model (GDV-40) with the exact same specs and dimensions, that can be just swapped out without too much hassle, and that would indeed be the easiest solution - except that they are quite inefficient (0.63), while costing $1690 each. Yikes. That made me think, if I'm going to pay this kind of money, I might as well look into better options.

With relatively cheap electricity (10c per kwh flat rate), NG prices going up, and hot summers around here in central Texas, it seems like a hybrid heat pump could be a good fit. The closet also seems to be originally sized for two atmospheric water heaters as it has 3 air intake grilles from the outside, and a 8" duct that terminates in the attic. As I understand, that should also help with heat pump efficiency during summer months.

So far, the best plan I could come up with is to replace one gas heater with an identical one, and the other one with a 40 or 50 gal hybrid (I'm currently looking at Rheem ProTerra); connect them in series with bypass and isolation valves. Why two different types of heaters? Memories of 2021 winter, when most of the city lost power for several days, are still fresh, and always having access to hot water even when electricity is not available, is definitely nice.

Any gotchas/caveats with this approach, or maybe there are even better options available that I'm not aware of, and therefore not considering? I'm open to suggestions and would love to hear y'all's input.
Also, I assume that it's better to put the more efficient hybrid heater to be the first one in the series, as it would be doing the majority of work, but if I'm wrong, by all means, please tell me so.

Thanks!
 
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