Can distilled water be harmful if used regularly?

[powderherface]

Hello! The water in my area is really hard, and the filter I own doesn't do the best of jobs. So I've decided to conduct an experiment: get my hands on pure (distilled) water, and use my 'weakly, weekly' fertiliser (which contains some Calcium & Magnesium) to balance this out. All my orchids are mounted, in a greenhouse environment.

This leaves me with a few questions & options:

1. in the 6 days a week, when I'm not fertilising, is there any biological harm I can do to the plant by only offering it pure water on those days?

2. would a daily fertilisation (at x7 the current dilution, in distilled water) actually be better & closer to what epiphytes may get naturally?

3. or perhaps, somewhere in between, e.g. every other day (further diluted x3.5), with a distilled water flush in between every water?

Would be great to hear people's thoughts & wisdom on this! I lack the plant science background to differentiate between these options.

PS: am aware of alternative pure-ish waters (RO, even rain water) -- but distilled was the easiest to buy where I leave.