Joining Dissimilar Metal Pipes - Out of New Well and Expansion Tank

Have just had a new well drilled and a line run into my small northern Michigan cottage. As the line enters the cabin, it branches off to a stainless steel expansion tank and then to a water shutoff and spigot. The piping where the spigot is connected is galvanized, and the spigot seems to be brass.
I want to remove the spigot (there is a shut off further up the line after the expansion tank) and begin to attach 3/4" copper which will then go to a sediment filter, water softener, and water heater.
Am I guessing correctly: The galvanized pipe has to be connected to a brass nipple somehow (not directly to copper ever), then you attach the copper to that? Teflon tape ok? This must be like a water heater connection right? Except that a union is used there (dielectric union) so the heater can be replaced again? Would I use that (dielectric union) in my application, remembering that there is a plastic sleeve in those unions - improvement over simple brass?

Thanks. Chuck

Yes, you are correct on everything, if I am understanding you correctly. There is also, a rubber gasket in the union in addition, to the plastic.

I would go with a brass nipple before I would the dielectric union with the plastic insert.

You can screw a brass nipple into a galvanized coupling on one side and a copper x female adapter on the other.

Consider replacing the galvanized with copper.

and would I use teflon tape where the brass nipple screws into the galvanized fitting, and then again where the copper meets the other end...or is pipe dope preferable? thanks!

You can do either or both. As long as the end result is that it does not leak, that is what is important.

The pipe dope vs Teflon tape vs both is on going. So pick one or the other or both and you'll almost always be right!