Need watering and sun advice; some plants struggling

TLDR: We moved and not that long ago I repotted crotons of varying size.

1. Where on a moisture meter should a croton be watered? All the way dry or just hitting dry? (When meter is near the bottom, where the roots are)

2. Is bright indirect indoor light all day followed by 2 hours of filtered (sheer curtain and glass) direct sunlight too much or not enough sun?

3. A new branch on an old, 1-inch-thick stemmed croton is struggling, and a baby leaf (only the third leaf on this branch) dried up and snapped off. Could it have been sunburnt? Dried up from lack of water? It looked fine and suddenly it was dry. Its young "parent leaves" look and feel fine though haven't been growing that I can tell. The roots on this plant are fine including some new, and I thought I was *overwatering* it but now I'm no so sure.

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So we moved, and I repotted a group of 9 croton plants (received all in same pot) and rooted cuttings (all with woody stems and healthy green and red leaves). I think my repot soil was initially too dense to I added more perlite. Anyway, some of the smaller plants have since died, and I can't be sure if it's root rot or just death: no new shoots, old roots just died. This happened also before we moved, too, before I could separate all plants into their own pots.

The plants range in size from 2 feet tall to the length of a finger, and they were repotted in pots that were just deep enough to keep them upright. Some of the plants now have beautiful new white roots, including further up on the stem away from older roots, and some have a larger mass of roots compared to others. I figure this is just how it goes sometimes? They were struggling with root competition for a while? The pot they were in and are in now are plastic with drainage holes and a saucer. We did a drench and drain every 10-14 days or when the top inch was dry, but now I have a moisture meter.

The Xlux moisture meter seems to work just fine. If I use the meter and measure where the roots are (mid-bottom of the pot), where on the meter should I water the plant? When it's mid-low moist, or after it hits dry?

It could be not humid enough for the plants, in which case there's nothing I can do because I can't keep a humid apartment. Outside currently it's 60% humidity, and it's so hot (there was a heat wave) that I moved them inside because I could tell they were sun stressed, didn't acclimate them slow enough. I mist them every morning and have been doing that, but maybe that isn't enough?

I should note I see zero bugs or fungus or anything other than springtails (I definitely checked what they were) in/around the plant. The leaves look good, some more than others, and I can definitely tell when a plant is struggling by their leaf position and firmness.

The soil I used is universal potting soil and brand new, and I made sure to mix it with the original pot's old soil because there was nothing wrong with it, but it wasn't that much because 9 crotons were in a 17-inch pot.

I figure part of the problem is repotting and rehoming and potentially heat and moisture stress, but nothing can be done about that other than make sure I'm giving them everything they want.

Thanks to all who can offer advice!