You are closest to the solution to the primary process and the fragmentation of the RepParty.
April 15 is a good start but let it run until Jun 15.
Then have the conventions between Jul 1 and Aug 15.
By bracketing the start dates and curtailing the lack of party discipline (my state first) the issue of disenfranchisement ceases.
Most of these posts miss the point that Rockefellar republicans are in the party at all and thereby making
RINO an incorrect term, it should be CINO.
Recall that the RockefellarWing is as liberal as the KennedyWing.
By not having any party discipline and purge the party of ALL LIBERAL factions (socialist/neocons by other names spells Commie/fellow traveler)is the cause of the current matters in force.
If that purge could occur then RINO would have come accuratcy.