I used macOS Mojave for years without installing dedicated WCH drivers. I don't think High Sierra is any different.
But you may also try the 2 second Urboot option to see if there may be a timing issue. BTW are you able to upload to your boards using Optiboot, Avrdude 8.0 and -c arduino? It may as well be an issue with Avrdude that affects your setup. The RTS/DTR pin(s) behaves differently in Avrdude 7.1 and newer, compared to Avrdude 6.3.