no. not essential to have water starts sorted prior to using harness and footstraps. In fact, i'd say learning to use these before water starting is better.
Obviously the windier (but not too windy) it is the easier water starting will be but having too big a sail will make life a bit difficult when water starting.
My advice is to just commit a whole day to water starting. 15-20kn is perfect
Water start, harness and footstraps, gybe. do it again.
So as you've nailed water starts you'll be wanting that new 130L board