Schefflera arboricola leaves turning brown and falling off

Raising houseplants is a very recent hobby of mine brought on by the pandemic. I purchased a mature schefflera arboricola in July and it sits near an east-facing window (no direct sun) supplemented with a grow light in the afternoons. I live in the northeast and the summer has been pretty humid, but the plant hasn't been exposed to any drafts or heat and I haven't gotten the foliage wet at all. I haven't repotted or inspected the roots since getting it, but the medium appears to be peat-based with a decent amount of perlite. It came in a plastic planter with a large drainage hole on the bottom. Knowing what I know now and considering the subpar lighting in my apartment, I would probably have put it in terracotta with a better draining mixture. All the plants I've gotten since are in terracotta with a mixture of peat, perlite, and bark and are thriving. The schefflera did really well in the first couple of months and was putting out a lot of new growth.

Sometime in the past month, I noticed a lot of the leaves turning fully brown and entire stems falling off. The leaves that fall off are pretty dry and not mushy at all, and it even seems like a precursor to the leaves browning is the stem begins to detach from the main branch. I thought maybe it was some sort of pest, but I watched tons of videos and read up on mealy bugs, thrips, aphids, spider mites, and searched everywhere on this plant (even with a magnifying glass) and could find no evidence of any bugs except a small number of fungus gnats that are mostly coming from other plants.

There is one branch in particular that has lost a lot of its leaves and has a part of the branch turning brown. I've included a picture below as well as an picture of the full plant. The leaves at on the floor have been there for about a week. The brown part of the branch isn't really mushy but I can scrape it off and expose more brown stuff underneath...the browning of the branch started from up high and not below. Since taking that photo, the browning spread up and down so I cut off the branch below the affected area today to be safe.

I'm having a hard time figuring out what's wrong - is it a disease or moisture issue? I gave it a very good watering a few weeks after I got it, then waited until the soil was dry before the second watering. I had stuck a finger into the soil all the way to my second knuckle and it was all dry, but the bottom near the drainage hole was still damp so I held off on the second watering.

Present day, I bought a moisture meter recently to get a better idea of the moisture levels (I know they can sometimes be inaccurate), and when I checked yesterday the soil was dry all the way through (which I was expecting based on how the top few inches felt). I gave it a thorough watering. I know browning leaves can be a sign of overwatering as well, but I've watered this thing about 3 times total over the course of the three months I've had it. Does this look and sound like root rot/overwatering? Is it possible my problems are stemming from underwatering/inconsistent watering or something more sinister? I'm pretty new to houseplants but I seem to have gotten the hang of knowing when my other - admittedly much smaller - plants need water. It seems to be pretty healthy aside from the one branch.

I'm planning to check the roots and potentially repot into terracotta and a better mix tomorrow. I didn't want to disturb the plant too much but it doesn't seem like I have much choice at this point. Can anyone offer advice as to what's going on with my poor plant?

I snipped a cross-section of the browning part and this is what it looks like. Not sure if this is at all helpful.