I don’t think you need to risk adding the Osmocote to the substrate - they can risk releasing too much ammonia into the water column.
What water column fertiliser are you using?
I do not remember any adverse effects the last couple times I added the osmocote tabs as far as the water column goes and leaching... I would imagine anything like that would be sucked up pretty quick by the emersed plants or absorbed by the substrate... I am using Thrive by NilocG Aquatics, and seachem equilibrium 3 times per week
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