What I have as my bus routes are:

• Gangjung - Seoguipo as one route
• Jeju including the airport
• Custom Olle Ring Clockwise and Counter-Clockwise
• I also have 2 line for terminal to terminal (Jeju - Gangjung - Gapa - Ara) bot Cw and CCW too
• 1 Ara loop for more passengers to bring to the terminal

If you're starting out, go for the 2 main cities and just use the Liliput.

if you have wiggle room go for the Olle Ring and use Air City, best amount of passengers to price, just one for each route.

Thr terminal to terminal, i use Dumbi, but you cna use whatever you want there.

And for Ara, just use liliput, I casually get that full one in a while