Not a cactus; looks like a succulent called Euphorbia trigona. If you do a web search for "euphorbia trigona care" numbers of relevant hits should come up.

I have that Euphorbia in my front yard, and it gets similar brown patches, but smaller and not as pervasive.

The one in the back looks like yours - photo from Jan2022.

Photo from Dec 2018. I don't know what the larger Euphorbia is, but it looks healthier.

These are in Coastal Los Angeles - I don't know where you are. If you are in Florida, there is a chance the plant is getting too much water.

Hi Lars! I think that one on the right is E. ingens, but probably something else. Nice garden.

L Evve,

I would suspect a number of things are bringing you excessive corking / slow strangulation of the plant. Here are some thoughts:

1) Pot's too small for the plant. If you want a (mostly) healthy looking plant, you'll have to start over with cuttings (easy, just must be careful of the white latex that can be caustic to dermal surfaces and terrifically so with mucus membranes).

2) Because the plant's roots have taken over the pot, there's no nutrition getting to the plant, and probably not any water in that strangleholded annular space - your plant's going to suffer anyway from that treatment, and, IIRC, cactoid Euphorbias are prone to corking / discolouration if rootbound, even more so than usual.

3) When you do repot, you might try an experiment of taking cuttings and whole sections / section of the original plant, as you'd get a larger plant sooner if you retain the stump and branches, or most of them. You'll probably want to wear appropriate PPE for the task of wrasslin' with that monster - take PARTICULAR care of your eyeware, as Euphorbia sap is known to seek out the vulnerable and the unprotected.

4) Given where it is, prone to drying out, I'd use a highly porous mix with additional feeding during its growth season. I don't know where you are tropically, but rainfall alone is probably not enough for a reasonably (not underpotted, as this one energetically is) potted plant in that pot.