They need repotting into porous soil and a larger pot. I’d guess the discolouratoon, which I think is cosmetic only and not indicative of rot, is due to the roots getting strangled.

ETA - there are a million and one soil opinions. IMO, you should have a gritty mix, but this is highly dependent on the species - there are very many tropical jungle cacti. The easiest way to do that is 50% perlite and 50% C&S soil, sifted to remove the major and semi-major bark chunks. From there, you could have endless variations, but some things should be used and some shouldn't.

To use:

pumice

decomposed granite

stones, depending on the size

leaf mold

scoria

coir

To not use :

sand, unless its particle size is large (bigger than 1/8 inch)

Thank you. The big pot was 2 potted cactus I planted into one bigger pot in the spring.

i love in Pa. they are in my condo in Florida.

normally go monthly but with COVID I haven’t been there since mMarch. normally a neighbor waters them every two weeks..when I’m there I put them on my patio for 1-3 weeks. In the sun and water weekly.

they are 25 yrs old and never had brown until last year. I’ve moved them to Florida 5 yrs ago.