Leaving the Republican Party only gives Uniparty members a greater advantage of numbers.
Ross Perot and Pat Buchanin proved a 3rd party is not likely to work.
The Tea Party and the Freedom Caucus proved that caucusing in a united front within the Republican Party can work to a point.
We just have to keep primary-ing the SOB's until alternative candidates win primaries. They can also run as Indepented in the General Election and caucus with the Republicans ... or whomever makes the best offer.
The greatestet threat to both parties is electoral irrelevance. They exercise power by controlling the primaries. If more candidates run as Indepenedents - and win - the party Establishments will be feckless.
It's a numbers game that can be gamed.