If this was the first year the plant spent the summer outdoors, then it is probably still just acclimating to being back inside. Growing conditions are different enough between indoor and outdoor areas that plants often shed some leaves after a transition since they need to re-adjust to how many leaves they can sustain in lower indoor light levels and lower ambient humidity.

We recommend removing the rocks from inside the base of the pot – they do not improve drainage, and in fact, they can make drainage worse due to a phenomenon called a “perched water table.” Essentially, due to how readily moisture drains through different substrates, they make the soil above them retain more water and risk drowning roots. A plant pot should have at least one unobstructed drain hole in its base (and only potting mix down to the bottom, with no stones or pot shards), and if sitting on a saucer to catch drips, the saucer should be emptied promptly after watering so any excess water doesn’t soak back up into the pot. Your plant appears to be propped-up above the saucer so it won’t sit in drained water, which should be fine as long as the item holding the pot up isn’t blocking the drain hole or wicking water back into its soil.

When plants shed leaves due to stress, it’s often the oldest/older leaves that yellow and fall off, which appears to be the case here. Dying leaves can develop some spotting due to minor infections that aren’t a threat to overall plant health because these microbes are only decaying the leaves that are already dying. We don’t see any indications of a pest or disease issue in the photos, which is good. We suggest you avoid spraying any pesticides (insecticidal soap included) until a pest is confirmed to be present, since exposing plants to pesticides (even organic ones) can risk phytotoxicity, which is tissue damage due to chemical exposure. Even pesticides labeled for use on plants for certain pests, and applied when needed, occasionally cause phytotoxicity for sensitive plant species. We don’t see indications of phytotoxicity here, but it’s something to keep in mind. Spider mites can thrive indoors with its low ambient humidity, so using a room humidifier if needed to raise humidity levels a bit can help suppress any outbreaks, as well as keep plants less stressed. (Even species native to drier habitats. Crown of Thorns can tolerate low humidity overall, but a bit of a boost above ambient levels in typical homes during winter may help.)

Has the plant been repotted in the past year or two? If not, it may benefit from fresh potting mix. Old mix tends to drain more poorly over time (and could also absorb water less effectively when watering is needed) due to degradation, and can also retain too many salts. In this context, "salts" are not necessarily sodium-based but can be any tap water minerals or old fertilizer deposits that the roots didn’t absorb and which build up over time and damage roots. By repotting with fresh mix replacing as much of the old mix as possible that can be teased off the roots, you can reduce any salt levels as well as improve drainage for the health of the roots.

Leaf yellowing for houseplants can also result from under-fertilizing, though typically in winter houseplants aren't fertilized since their growth has paused due to light levels being much lower than in summer, even for sunny window exposures. If a plant wasn’t fertilized much during the past growing season, though, then a light dose of general-purpose fertilizer (focusing on a formulation containing more nitrogen, N, than the other nutrients in the N-P-K ratio, if possible) may help it retain more leaves. It can’t reverse yellowing on any leaves that have already changed color, but it might help the plant to support more leaves overall during the “lean times” of low light and short days while indoors for the winter.

The various moisture meters for potted plants on the market may or may not work well enough to be accurate…we have not scientifically tested them for reliability. Generally, watering needs can be determined by feeling the soil about an inch or two deep into the pot; if somewhat dry to the touch at that depth, a plant typically needs water, or will soon. Granted, feeling soil in a pot whose plant has very thorny stems may be challenging or undesirable, hence use of a meter, but you may be able to judge the meter’s accuracy by testing with a finger a few times at first to see if the meter seems to indicate enough dryness at the right time for watering. In general, Crown of Thorns does best with a bit more regular watering than most other indoor-grown succulents, though still prefers to get a bit dry between waterings so it's not constantly moist. University of Wisconsin Extension has an article about this plant with more care tips.

As an aside: given how long you've been growing the plant, you likely already know this, but avoid exposing the plant to light after sundown since they require short days (and nights with uninterrupted darkness, like Poinsettias) to flower well. Even turning on a bathroom light for a few minutes after sunset could affect the plant's flowering behavior and suppress blooming, since it can basically trick the plant into perceiving that it's a different season.

Miri