The 21 of 31 is not a Senate Rule it is a Lt Governor ploy. The power of the Lt Governor comes not so much from the Constitution as it does the Rose Bush Debate that opened every session for the last 40 years, and accounts for the 2/3 vote needed to bring something up.
In talking to the two Reps last week they were convinced that to revisit re-districting they would need a two thirds vote in each house. I have no further information.
I think 130 years of "conditioning" is in play with regard to what I'm hearing on this so far.
The fact that we will now have control of ALL the leavers of government hasn't yet sunk in methinks!