I'm kind of confused on the question since you said "expansion tank". Which I guess is used because of a check valve - but that's if you put a check valve to the cold water back to the city, which isn't really part of a recirculation installation. A check valve for the recirculation should not need an expansion tank.

A heat pump water heater will not work well with this mixing valve - just pipe the hot water direct to where it's needed, don't mix it with cold. A heat pump doesn't really want to make very very hot water, unlike fuel heat. This valve is meant for you to heat the water extra extra hot, then mix it. It's used when you have a small tank, and a lot of usage. It wastes energy, but gives you more hot water.

It's just a bad idea to mix with a heat pump which much prefers lower water temperature.

And using this mixing valve with a recirculating pump can't possibly work. This valve adds cold water, but a recirculating valve circulates the same volume of water, and does not add any. There's simply no way that it can work - it's impossible. Do you get why?

Are you planning on having the recirculating pump always on, or do you prefer to press a button and wait a short time for the hot water? I ask because if you prefer the first type, you really want a dedicated return pipe for it. The second type can use the existing cold water pipe for return, but even then works much better with a dedicated return.

Without a dedicated return line you never have cold water, the cold water is always warm because when you turn on the cold water, some hot water flows via the pump, and to your faucet.

The expansion tank should be connected directly to the hot output of the heater before any mixing valves or recirculating stuff.