Excess calcium CAN (doesn't mean will) effect kidneys causing issues such as gout and even failure. It is most likely to happen in birds that are genetically predisposed somehow or have also been fed too many low nutrient treats such as scratch.
Some roosters live to ripe old ages even though they never ate anything but layer.
That isn't my personal choice... and so I feed Purina flock raiser with 20% protein and oyster shell available on the side. I also have many stages of bird including molting, growing, brooding, roosters and layers.
The ONLY thing layer has is LESS protein and more calcium... basically. I NEVER switch to layer and you don't have to unless you like. Dual purpose birds such as Marans, Orpington, Rocks or Reds will do better with at least 18% protein.. as 16% in most layer is about the minimum needed to sustain a light bodied layer like Leghorn. That being said... energy (or calories) only come from three sources... Protein, fat, and carbs including fiber. I choose for more of my calories to come from protein as I consider it a better source of nutrients including amino acids... though the amino acids are added into all the feeds.
But there is a limited point of return... too much protein could also cause a different type of gout and ultimately failure... though I believe it would have to be very high. So, the birds can only process so much protein.. after that it's processed out as expensive and extra smelly waste.
Your current thought to just add OS on the side of your grower is what I would do. If the grower is pricier than layer... it's because OS is cheaper than protein. For me it ends up washing out in the end and most my birds aren't ingesting the extra 3% or so of calcium every day.
Should you decide you don't wanna switch to layer... just mix down the bag you have like you already are... the whole kidney thing is non laying birds fed an excess of 3% calcium LONG term.
Also, HEY... welcome to BYC!
