It needs more sunlight. It hasn't been getting enough sunlight for a long time. If you increase the amount of sunlight then do it gradually. Sunlight increased too quickly can sunburn plants.

Thanks, Polypompholyx. I've made the mistake of too much sudden sunlight before and got worried, as it's mostly been inside or shade. But I think's it time for a slow transition outside.

Also time for a repot, when you can also take cuttings / pups to prop them separately. At any rate, I'd guess all parts of the plant / the plant would benefit from fresh soil, which would, with Poly's advice, greatly improve its lot in life.

Here’s a little info about your cactus🌵.

UPDATE:

Thanks for the suggestions. After some time outside, here is the cactus this morning: