I am an experienced Armada 1 player who used the Millennium Project mod extensively. Are there any single player mods that rival the Millennium Project mod for Armada 2?

If you mean for the campaigns, then no. For skirmish there is the Fleet Ops mod: https://www.fleetops.net/download
You can also try my own take on the Fleet Ops mod: https://www.moddb.com/mods/fleet-ops-roots/downloads/fleet-ops-roots which gives you gameplay more in the style of Armada I, so with fewer ship classes and smaller fleets of ships that last longer in a fight (in vanilla Armada II or Fleet Ops ships are quite disposable compared to Armada I), you also don't even need to own the dic-version or the GOG-version of the game to play my mod.