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Has the temperature changed to below 50 degrees F (10 C). A drop in temperature can cause the black, dropping leaves.

It could be due a pot change as you say. You can refresh the soil and trim the roots a bit.

Here is more info:

https://pss.uvm.edu/ppp/articles/schefflera.html

https://www.gardeningknowhow.com/houseplants/schefflera/repotting-schefflera-plants.htm

It sounds like you know what you are doing and from the photo and description of your practices the plants seem to be doing well so far and are likely to survive.
They need to be kept moderately moist, but you may not need to water every day. You don't want to encourage root rot.
You might also consider a product that helps plants become established that you add to the plant water once a month during the growing season. Just ask at any garden center.

https://www.gardeningknowhow.com/houseplants/schefflera/outdoor-schefflera-care.htm

This should be expected with a rapid change. They are sensitive to this. The best thing that you can do is to put it in a brightly lit area, and only water once the top 4 inches of soil are dry, completely.

Make sure that it stays warm. Anything approaching 50 degrees will start to make them drop leaves. Dropping leaves is also due to the drop in humidity and light, in indoor conditions.

Keep it alive through Winter with light and warmth, while feeding on occasion. Once you can put it back out, you can prune and treat it as normal. It will recover once it has the ideal conditions that it wants back.

This article will offer help on the care of Schefflera: https://www.gardeningknowhow.com/houseplants/schefflera/schefflera-plant-care.htm

I would make sure to keep an eye on that! It looks like the very beginning of a fungal/bacterial infection.

I can't really see if your container allow extra water to flow out, freely, or not. If it does, I would let the plant dry out down to about 3 or 4 inches before watering again, instead of watering on schedule. This will offer a more accurate reading as when to water. Overwatering is the leading cause of this type of infection.

Here are some articles that will help:

https://www.gardeningknowhow.com/garden-how-to/info/using-fungicides-in-garden.htm

https://www.gardeningknowhow.com/garden-how-to/info/applying-bactericide-to-plants.htm

https://www.gardeningknowhow.com/houseplants/schefflera/schefflera-plant-care.htm

Schefflera leaves will burn if they get direct light, but that usually appears as yellow spots that turn to brown leaves. Most problems are often related to over-watering. I would cut off any diseased looking leaves at the top and prune it down a bit, then treat it with Neem oil. Make sure it's not being overwatered. Wait till the soil is dry 3/4 the way down before watering again.

https://www.gardeningknowhow.com/houseplants/schefflera/schefflera-plant-care.htm

I'm inclined to say a slight bit of overwatering is the main problem. Other than that- Leaves with distortion can be caused by a pH imbalance or cool temperatures.

At this point, providing correct care should be enough without serious treatment. This article will give you some information on the requirements of this tropical shrub:

https://www.gardeningknowhow.com/houseplants/schefflera/schefflera-plant-care.htm