separate colors from whites
This is especially necessary when washing warm/hot, as many dyes dissolve at higher temps.
You can probably buy washing machine cleaner liquid that you run on an empty cycle.
Unnecessary. Leave the door open to air dry between washes. If it ever smells (otherwise twice a year) use 1 Cup vinegar OR 2 Tbsp citric acid (dissolved) in an EMPTY machine on baby/sanitize/hottest cycle to clean any molds or limescale. Can also use 1 C of bleach (dissolved).
Fabric softeners are a waste of money if you ask me
They offer no meaningful benefit. Add 1/2 cup of vinegar to the softener dispenser every wash. It's a natural softener that inhibits mold and neutralizes odors (both clothes and machine).
ALSO, hang/dry clothes within a couple of hours. If they aren't dry within 24 hours (36-48 max), they will start to grow mold and smell no matter what.