Teeny bantam struggling when laying

[Fwoof]

I have a bantam who's always been much tinier than my other banties and on the weaker side in terms of balance. She's been laying for a few months now and her eggs are very small and she has difficulty laying them. I know she's still just starting her laying, but twice now she grew suddenly lethargic, laid a shell-less egg, and perked back up to full normality. She grows lethargic often before laying an egg. One time she even laid a shell-less egg with blood streaks on the membrane. She is fed layer feed and has constant access to oyster shell.
I have both vitamin D3 and calcium citrate powder from bulk supplements, and I was wondering what a good daily dosage would look like and if it would help at all. I gave my Sebright (Stormy) with laying issues in later life 1/16 teaspoon of D3 and 1/16 teaspoon of calcium (both powdered form) mixed with oatmeal daily that she ate willingly. Her eggs improved, but my runty bantam Dusky is small and fragile and I don't want to overwhelm her.
I've also heard some fish oils are good so will feeding her a small amount of canned sardine in water reap any benefits? I heard phosphorus is good in small amounts but the last thing I want to do is overfeed it and cause a whole new set of problems.
Any tips are greatly appreciated by both me and little Dusky!