Over watered, you have a saucer under it, don't leave any water in the saucer. The pot is too big. More light.
If the bottom is not soft let it dry out
Your medium looks "organic". Try cinders or a cactus mix with a lot more perlite. I live in a wet place and I water every day. I use inders and rocks.
Get a pot that breathes. Insead of glazed ceramic, get a terra cotta pot and only big enough for the roots too fit. Cactus don't need all the extra space that is going to hold more water. I prefer the azalea pots that are more squat. If you have to use a saucer, get a big one and use pot feet to keep the pot out of the water. Drain the saucer of any water, cacti don't need humidity.
More light, more air
If the bottom is soft, it may be rotting. Cut off the rotting part and make sure you leave only healthy tissue. Use a clean sharp knife. Let the end callus of and dry for a few days or a week then repot as above. Cactus food once in a while. The easiest way to know when to water is to do the same thing that we do for orchids. Pick up the dry pot and feel the weight (or put it on a scale). Water the plants thoroughly until the water is running out the drain hole. Drain the pot for 30 minutes, pick it up again and feel the difference. When the pot feels light and ready to water, wait another day and take it to the sink; water and drain again. Cactus are high light plants so you need to have a light over them or in a bright window or better yet outside when the weather is nice.
Cactus and succulents are sponges, some of them can latch on to even the moisture from the air so you have to water sparingly, make sure the soil drains and it does not sit in water. They need a lot of light. Some succulents like lithops cannot be watered while they are shedding their leaves for about 3 months. I know, I killed one because I forgot to bring it back inside one night and it rained. I know someone who has an window collection of mini cacti and he actually wipes the succulents with a paper towel after they are watered to remove any excess water from the plant. Cactus can tolerate being underpotted but will rot if you have too much wet media in a big pot.
Happy gardening in Hawaii. Gardens are where people grow.