> Maybe a shorter duration(<5yrs) patent(for lack of better word) for unapproved generics might do the trick.

There already is something similar - the NDA exclusivity period. You get 3-5 years where the FDA won't approve any other versions of the product.

https://www.tandfonline.com/doi/pdf/10.4155/ppa.14.30

But ambroxol isn't a decongestant - it supposed to help with phlegm.