So I'm looking at my formula for combat wildshape HP. It seems at lower levels, the combat form's HP is a bit higher than the average HP of the available wildshape options. But at higher levels, the combat form's HP is a bit lower than the average HP of available wildshape options. So I'd say I eyeballed that pretty well.
In addition, earliest wild shape forms that offer huge size is 9. When a combat form can become huge, I'd say they should also gain double damage dice for their attacks. But with the existing way moon druids gain abilities for their combat form is connected to ASIs. I feel like offering huge size at level 8 is too early, so maybe as an option at level 12 makes more sense.
But then again, the ability to take the form of an elemental at level 10 for the cost of 2 wildshapes breaks trend with the moon Druid's normal wildshape progression. Technically, using this ability they can take the shape of a CR 5 elemental at level 10. Normally, a CR 5 creature wouldn't be available until level 15. Plus all the immunities and resistances, as well as attack forms, make it difficult to incorporate these abilities into my proposed adjustment to wildshape.
So, as I see it, perhaps at level 10 we allow 2 new options that are templates to the combat form. One would be a huge version that doubles damage dice. The other would create an elemental version of your combat form, but simplified. You'd gain immunity to one energy type associated with your chosen element at the time of wildshaping (and even perhaps getting healed when subjected to attacks of that energy type), and also a special ability or two tied to your chosen element.
As I see it, doing it that way wouldn't require two wildshape uses, since you would get many fewer immunities and resistances. This can also be interesting by making it easier to in a sense create new elementals related to other energy types (acid, poison, lightning, cold, ect).
But I'm just kind of thinking out loud.
I'd also say that starting at a certain level, you could raise your combat form stats above 20 (to a max of 24 maybe).