If I just want an overview of Seoul, is a full day-and-a-half enough?

I am flying to Tokyo and will be staying for around 9 days. In the middle of that stay, I am planning on flying to Seoul. I am debating whether I will stay 4d/3N or just 3D/2N. It will be my first time to Korea and there does not seem to be as much to see or do over there as there is in Japan. (Been to Japan multiple times and I never tire of it.)

Yes. Nothing to do here; don't come.

I am just back from 5 nights in Seoul as part of a wider Asia trip. This was my first visit to Seoul.
Before hand I had watched plenty of YouTube videos but not really settled on what I wanted to see or do and ended up winging it day to day - this was a bit of a mistake, in that I should have spent a bit more time making a list of places I wanted to see and also researching where they were in relation to my hotel in Hongdae.
I found it similar to my first visit to Tokyo 10 years ago where I feel I only scratched the surface of what there was to see and do.

How was the Korea visit?

Flew from NRT-ICN-NRT specifically to try out OZ's A380, especially the upper deck (I'm an AvGeek). Loved the service and the plane, although OZ as an airline and its amenities are tired. Oh wait, they are bankrupt!

ICN, I think is not an ideal airport especially when visiting for a short time. I arrived on a Monday afternoon, and only staying 4 days/ 3 nights. Waiting for a taxi to Seoul at ICN took forever (line snaked half a block), then the commute, then the traffic jam.

I hired a private guide the next day. Whole day. It was great. I burned off a lot of calories, got to learn how to use the subway and saw some 'essential' attractions.

Third day, I was too tired to go to Lotte World. Just spent shopping around Gangnam area. I stayed at the Intercontinental. I ate some traditional Korean lunch at Asunje. Highly recommend it. Beef BBQ was great at Two Plus Godeum Samsoeng.

Fourth day, I had to leave early for my flight back to Japan. ICN immigration was a mess early in the morning. Loooooooong lines - a full hour wait to get served. OZ business lounge felt like a bankrupt cafeteria.

I instantly regretted my decision not to buy some in-city purchase, but airport pick-up duty free items. The 50ml Jo Malone was only about USD50 when paid for at the city department store, but for airport pickup. (I was afraid I will not make it to the gate pickup area). At ICN itself, price was the regular USD65. Boo.

IC Seoul Grand Parnas had a nice location and the junior suite was nice but I felt it was overpriced. I enjoyed their breakfast although I found it strange that gochujang as a condiment was nowhere to be found at the buffet with the abundance of Korean dishes.

If ever I return to Seoul, I would probably rather fly into and out of Gimpo. And I will buy the complete set of Starbucks limited edition exclusive Korean mugs.

A lady friend of mine told me that Korea is known for its beauty products and treatments for both men and women. She encouraged me to find a beauty spa for men. Honestly, as much as I like perfumes, I am not that vain so I forgot about it. LOL