Dilemma with duck laying soft-shell eggs

Hi I hope I can get some advice and wondering if this is common. I have a duck that's almost 4 years old. She started laying for the season and laid 3 or 4 normal eggs. Since then, she has laid 3 soft-shells, one no-shell, and one super tiny egg.
She is not eating the oyster shells as she normally has in the past. She also might be exhibiting a little bit of lethargy, BUT it also might just be broodiness setting in, which is common for her. This morning, she remained in the nest even though she had already laid an egg last night.

Today I have bought some soluble supplement ("Calcium Plus" for birds) containing calcium,vitamin D3, and magnesium.

DILEMMA: should I
a. allow her to go broody (if that's what she's feeling). This would stop egg production which is good because it's not safe to be producing soft shell eggs, right?
b. stop the broodiness. This would keep her in a more "normal" behavior mode, which would help me determine if she is sick and also if the supplement will help.

So far today, I have kept her from the nests and she is behaving pretty normally.

BTW I use Mazuri Maintenance and greens and some mealworms, and she has occasional access to a big yard with bugs and such. Could try Mazuri Breeder?