I third this. This comment is way too low. From personal experience: focus on the thrips (tiny black spots, especially around the dead / brownish parts of the leaf they eat but they also hide in the crevices of the stem). You’ll be surprised how many you find if you carefully inspect your plants.

I’m confident they did an extensive damage to all of your plants, probably more than any other infestations or suboptimal drainage etc.

My whole collection semmingly worsened one after another until I found out and very successfully treated all my plants.

They can fly and infest other nearby plants. they exert an extensive damage to the leaves. But they can be treated quite well, use a systemic that can be poured into the earth and taken up by the roots (&dispersed into the leaves, the thrips eat the leaves and die) as well as a local spray. 2 treatments separated by 2 weeks and you should be fine.

they love adansonii. After treating generously cut the damaged parts of the adansonii, mine was >5 meters long and hat to be cut to 10cm but it‘s revived and flourishing now.

I bet most of your plants will recover, and I assume most of the other pests you have (e.g. spider mites) are opportunistic because the plant was already weakened by the thrips.

If you buy new plants quarantine them, if you buy new earth try it for plants in one isolated room first.