So I am a partial mash brewer. My water is high in rust. So last couple of brews, I've brewed half with distilled water and half filtered water and my hoppiness of brews have improved and definitely taste better . I was wondering what would happen if I switch to all distilled water? Would I have to add anything to the distilled water for brewing? Thanks in advance for help.
As far as general advice, I've seen plenty of places (and it makes sense to me) that if you're using DME or LME then the maltster has already made extract for you with treated water (e.g. minerals should be fine). That means you shouldn't need to really worry about this with extract.
In partial mash, it seems you have a batch that's part all-grain (which you need to adjust) and part extract (which you shouldn't need to adjust as the maltster did it for you).
I'd figure out your proportional volumes (i.e. 3G of wort from extract and 3G from mash), and then use a water calculator like Brewer's Friend or Bru'nWater for the partial mash portion. That will help you determine the appropriate amount of salts (probably Calcium Chloride and/or Calcium Sulfate) plus an acid addition to hit your appropriate mash pH.