Here is how I post to only one #nostr relay at a time.
Tap + in #Amethyst
Type shitpost
Tap the circle relay icon in the top middle
Select the one relay you want to post to
Tap save
Tap post
If you only select the same one relay to reply, it doesn't broadcast any further.
This note is sent first only to Derek Ross (nprofile…jrmw) #nostrelite relay