Although I wonder if Taiwan under a hardcore Guomindang dictatorship, it might justify its dictatorship on anti-communist grounds and be hostile, still claiming to be readying to reclaim the mainland.
Also, I wonder what tensions would build up internally in a society still under dictatorship for another decade or more. And tensions built up in the US-Taiwan relationship over it being undemocratic. A lot of the argument for getting closer again to Taiwan after 1989 was that it was democratizing, it wasn't just geopolitics like in the 50s and 60s.
Likewise, might Beijing worry about a continued South Korean dictatorship being more aggressive. I would think a regime like Chun Doo-Hwan's in the 1980s would be more anti-North and willing to invade than the North compared with any democratic regime, especially of Kim Dae-jung's party pursuing a "Sunshine Policy", rapprochement with China and determined to punish opponents have having "Japan-collaborationist" ancestry at this point. Plus, there's also a matter of South Korea's economic prospects an societal health under a perpetual dictatorships that probably only gets to stay that way through periodic assassinations and bloodlettings.