Whatever many faults Trump has, ruining the GOP is not among them.
The mainstream media can tell you Trump turned most Republican politicians into lying sycophants (in real life I never met anyone who uses the word sycophant)all they want but in real life there is little difference in the typical Republicans politicians behavior in 2015 and 2020.
As far as turning Republican voters into conspiracy theorists it was the Democrats who spent years on Russian collusion and other such conspiracy theories and indictments. (I do agree that many people have gone nuts with Trump Derangement Syndrome but for the most part they aren’t Republicans and it was the media, not Trump, who caused that syndrome)
As far as the general state of the GOP after Trump goes. Unlike twelve years ago when Obama first won noone is writing the obituary for the GOP after Biden won. No Republican presidential candidate in the past sixty years has gotten such a high percentage of minority voters. If anything Trump has shown that the predictions of the GOP demise due to the decreasing percentage of white voters is not true