Much simpler, IMHO

single letter = sound

letter + z = "hissing" version of the sound

letter + accent = soft version of the hissing sound

letter + i = same previous item, but caused by "i"

rz = legacy, czechs still pronounce it as a different letter

This is how I understand it as Ukrainian

Westernmost Eastern Europeans would do anything but use the actual script that makes sense for their language. How hard is it to just use с, ш, щ, ч and ц like civilized peoples.