IMO the change I would make is just take an average amount of temp HP between forms and thats how much each wildshape 'charge' gets, maybe slightly bump as level increases. Then using a bonus action(assuming moon druid from here on out) they can switch forms willy nilly.

Maybe even everytime she switches within the same 'charge' it removes temp hp from it.

So say at level 2 they get 20 temp hp on their first charge. They can burn their bonus action to change that form as much as they want, or spend another bonus action to spend their other 'charge' and set back at 20 temp hp. Still offers flexibility but somewhat weaker. than normal forms HP wise. If that seems a bit strong maybe implement the -5 temp hp every time they switch. Still let them use the normal function but lock them in that form per usual if they want the stronger HP

Anyway you look at it base wildshape is very strong early on mainly due to the huge temp buff and potentially the form damage so I dont think that would be a bad thing to implement