Finally a really good article in the topic that everyone seems to get wrong in the technical level. There's very little valuable human-readable documentation about the actual inner workings of Atmos, but you got most of it on point, only one and a half things need a bit of clarification.
"Are there some places where you can't place objects?"
Yes, you can't pan perfectly up the screen in a reference room, since the top fronts are not in the screen. The renderer's speaker locations completely differ from the recommended placement, for example, they're at the edges of the box that simulates the room, sides are perfectly on the side's middle, and all tops are above a ground layer speaker. The inner channel locations are in table B.10 of ETSI TS 103 420 V1.2.1. In cinema, where the inner locations match with the real ones, the tops are present for each or each other (since newer rooms) surround, fronts are not included.
"the bitrate used for objects is “a lot lower than you would think,”"
There's no such thing as Atmos bitrate. If you mean the few bytes that encode the JOC matrix per frame, then yes, it's <3% of the overall stream size, but the issue is not that. Atmos is disassembling each full bandwidth channel to 10 bands, and mixes these 50 or 70 bands to new objects. The Atmos metadata only contains this mixing matrix, so the actual audio is coded with the other channels. What you hear when you mute the bed/ground channels (that's possible with a single click in Cavern), is the result of non-perfect alignment (not enough bands, only a few bits per matrix element). There can't be more bands, as this process is so slow that it takes 160x more performance to render this than regular DD+. The Fourier transforms needed for the QMFB transformations in the JOC matrices can't be sped up, since the DFT equations are distorted in such a way that FFT can't be used. The performance for decoding DD+ Atmos is about 6-7x real-time on a Ryzen 5600. Real-time isn't even possible with hardware from a few years ago, that's why external ICs were always needed in home cinema applications.
