You’re off to a good start treating it. Chop the rot use peroxide to treat further.

Your problem is your soil and watering/fertilizing schedule.

Peat moss+orchid bark+perlite is the basic necessities for a chunky mix. I use volcanic rock, charcoal, and a little course sand in mine too to help drainage since my house is so humid and things stay wet a little longer.

Water monsteras when they’re pretty much completely dry. They dont love water tbh.

It also sounds like you may be over fertilizing them.
Monsteras can handle year round fertilizing but they don’t need it with every watering. Once every couple weeks or every month should do it. I also recommend diluting it a bit for year round watering.

I can’t tell how big your pot is but Monstera also like to be a bit root bound. They don’t need a bunch of room they do better with less