Some advice:
I tried to change 50,000won or buy coffee with 50,000won. Nobody in the station had change. So eventually I had to top up the Tmoney card using 40,000won (min if using 50,000 top up machines) then the 2nd card is 10,000won. Well, the card can be used to pay for items in convenience stores so no issue with that.
Nobody does take-out udon (cos kid sick) so had to resort to food delivery to have a nice udon delivered.
Olive yong is everywhere. So don't need to go to the main store in Myeongdong. The experience is so bad.
There is a kyoja in Myeongdong, (selling only 3-4dishes) with raving reviews insta etc. But it was just ban mian with lots of msg. And squeezing with the crowd in restaurents.
Pharmacies are quite common and some of them speak English and very helpful.
The google translator on my Samsung (holding the home button) is a life saver. On/off there are emergency messages of missing old man, or strikes at railway etc that I use to translate. Similar the food delivery app also use the home button...
If you have 4 pax or more. It's easier to travel by klook airport transfer instead of the 6018 bus.