Very fussy puppy – any suggestions?

Hi! We have a 5-month old Havanese puppy (we've had her for 3 months). She's lovely, cuddly, likes training, she's playful and does puppy stuff of course (going through teething, needing constant naps & shorter bouts of exercise).

The one thing that's been a challenge (probably even more than potty training as she's been doing so much better in the last month) is her meals.

We feed her 3 times a day, 8am, 2pm, 8pm approx. because sometimes she'll be sleeping or we'll be having a walk before her meal.

The breeder gave us Royal Canin Small Breed Puppy Chicken. We bought the same and she had it for a couple of days and then she started rejecting it. We tried to wet the food, etc. but nothing.

We wanted to stick to WSAVA-approved brands and went for Pro Plan Small Puppy Chicken. We mixed it with RC for a while, but we had to wet it, hand feed, sprinkle treats and all sorts to get her to eat. Then, about a month later she started rejecting it completely.

Then, we ordered samples from Platinum and she liked it, it was the first time she had a bowl of kibble without any decorations. We ordered a bigger bag and in less than a week we had the same pattern. Having to wet it or similar.

Every time we did follow the typical advice: no treats or just the few ones needed for training and given at least 2 hrs before her meal. Leaving the bowl for 15 mins and removing it. Also not upgrading the food immediately after a rejection. The assumption is that puppies won't starve themselves, but turns out ours did and we had to take her to the vet because she was lethargic and they had to give her calcium (luckily the investigation showed no other underlying health issues).

After that we had to make sure she ate no matter what. So we started mixing with toppers (salmon, trout mousse from Fish4Dogs, and puppy complete wet food to keep it healthier). And that went well for about 3 weeks, until she started rejecting everything.

We know she loves fish, we can tell she's hungry… but we cannot make her eat her meals. I think she could be developing some anxiety with food but are not sure. The vet said that it could just be her personality and that we'll have to offer different foods over time, emphasising that she has to eat.

Any advice, recommendations or support words are welcome.