Hmm…I guess I see what you’re saying about tap, but even then, would it be safe? I posted a new thread where I posted my city water report (can post here if you want). Biggest thing I see is chloramines but I’d think Prime would be sufficient if the rest in the water is fine.
As for RODI water, that leaves out top of water though? I would get an RODI unit and might but my concern is how fast I’d burn through filters
The rate of top-off for evaporation depends largely on whether you run open top, or have a cover lid. Most on here have more-or-less open tops, but it's not a necessity for success (for an example, Reefbuilders.com has their Red Sea nano running successfully for over a year with a glass cover and very low evaporation rates).
https://reefbuilders.com/2021/05/03/500-day-update-of-the-one-day-nano-reef-video/