Schefflera Arboricola has brown spots on new leaves

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The two main problems I am having are brown spots on new leaves and some new growth turns immediately black and falls off. When I water this plant I try not to get any of the leaves wet and I try to have a couple days of complete dry soil before watering. So I don't really understand why new leaves are forming brown spots and eventually dying and falling off. Some leaves keep growing with the brown spots and mature with them.

I have also noticed that the trunk has started to shrivle slightly. I have seen this before on some of my other scheffleras and I planted them in inorganic drainable soil which fixed the problem. For this plant however I would assume the problem would be overwatering (even though I feel like I am underwatering), or it has a fungus problem. Repotting this particular plant would be difficult because it is growing roots over rock for about a year now and I don't want to mess with the roots just yet.

Has anyone experienced similar symptoms with their plants and do you have any suggestions on how to get rid of the brown spots and establish a healthy looking trunk?