I do not grow cacti, so can't speak much about specific watering frequency. But I expect 1:1:1 gritty mix indoors would be too wet for a desert cactus.

Best to increase the amount of grit and decrease the bark or napa. Make sure you sift out the tiny particles.

An 8" pot is quite large, but I suppose it depends how fat the cactus is. Moving up just an inch or two in pot increases volume significantly (often 50-100% more).

To determine watering frequency, I recommend caring for it yourself for several weeks, if possible. For friends I often use a different/less demanding mix and it takes me some time to figure out the frequency (or reasonably closely).

Thank you. I read that the Cephalocereus tends to have a deep tap root, and given the height I thought the depth of the larger pot would help. I now understand it was likely root rot that caused the stability issue (and the crooked growth, which put significant force on the roots). I will post a picture shortly.

The crooked bend of the cactus is a challenge. I was planning to reorient it in the pot, but I'm not sure how far I should go on that, because of how it will angle what is left of the roots, etc.

Rocks may help secure/prop the cactus up while the new roots stabilize in the pot.

Depending on the store return policy or your need for pots, having one size down just in case may be a good option. If the taproot is much smaller than expected you are still covered. It can be repotted in a few years again if necessary.

Hope that helps!

This cactus has a lot of curves in it. There is approximately a 90 degree, and then also a slant. My thought is to plant it so the main body is vertical, and I have built an adjustable support for that. That will create a significant angle at the bottom. Turning it like that will tend to put some of the cactus under the soil. Will that be a problem? What should I do?

The new pot really isn't that much bigger than the current pot. I am still uncertain what to use for soil. I have 8822, perlite, coir, and a good source for pine bark.

Photo was not what I expected to see! Hopefully some others with better cacti knowledge can help.

I'd slice and dice that one and start over - its unnatural growth is a danger source, and for aesthetic reasons as well I'd get that into more suitable pots - two to three cuts should do it. Set aside to dry, making sure you've used a hygienic knife, and reroot the sections. You'll want somewhere warm to do so. Don't water the cuttings (unless the bottom one has roots). Mist only, but I'll give you more advice as you need it. You'll need deeper pots and some sifted potting soil, along with that other stuff (except for the bark, which is not needed here).

Thanks for the help! Hmm. Cut it up. I can see where that would solve many issues. I need to find some good resources on proper technique. I've done other cuttings, but not cacti. I'm doing some searches now, and collecting links to read.

How much length should I retain on the top most section?

So, to get started, slice it, and lay the cut sections horizontal for a week or two? Is there a guideline for pot depth? What sort of soil should I be thinking about for this initial phase? Thanks again!