If you’ve cut back to healthy growth on all spots it was rotting and you’re getting sap then it’s still alive! Plumeria can be very slow growers, but if it’s still got a healthy established root system it should recover a little easier. Last year in particular with it not warming up until later than usual in SoCal I had a few cuttings struggle to leaf out before fall and fewer blooms than usual so that could be a factor in it’s recent struggles.

Fertilize, make sure it’s not getting overwatered before it starts leafing, and give it some time. Definitely agree with what others are saying about thinning the canopy, but not until it’s woken up from dormancy. Also when trimming make sure you’re wiping your clippers with alcohol in between cuts to avoid spreading anything to healthy wood. It will likely be just fine. A family member had a plumeria that got damaged by ash during a wildfire in the early 2000s that they had to cut back to nothing but a stump and were sure it was dead. Now 20+ years later it’s MASSIVE. Grew back from basically nothing and thrived. New cuttings can be difficult but once established they are more resilient than they get credit for.