Tracycle, with the greatest of respect to Richard and this site, it is a low traffic site so my first bit of advice would be to cast your net wider for research.

1. CrazyGuyOnABike is the grandaddy of Bike touring Journals and a great tool for doing research. My strong suggestion would be to avoid the fora there. Here's a link to South. Korea CycleBlaze is another, but without the fantastic search tools that CGOAB offers.
2. Depending on where you are in the world, you may want to consider other cycling fora with a touring/adventure section. A good Google search will help a lot. Remember, some of tthe best of hem may be in foreign languages but Google can help with that too.
3. As always, YouTube can be useful and a simple search on the likes of Amazon incorporating "Bike tour" & "South Korea" will probably throw up some relevant links. Also, the likes of Komoot and RWGPS offer a "route search" function where you can locate other routes, planned or completed, by other users.
4. Finally, a good map shop may be able to help. Stanford's, in the UK is recognised worldwide as a source of maps and guidebooks. Personally, I like the fietsvakantiewinkel. It's Dutch, but all the staff communicate excellently in English, all have pretty extensive touring experience and it is a store dedicated to bike touring. If I was you, a friendly email to them would be my first step.
5. Good luck! It sounds like a wonderful, interesting adventure!

Thanks so much for all those excellent suggestions! I really appreciate you taking the time to provide such a thorough and thoughtful response.

I've done some initial research on CGOAB and it does look like a goldmine of trip reports and routing ideas. I'll be sure to spend some quality time digging through that site. The tips on Komoot and RWGPS are also great - had no idea about the route search functionality, so I'm excited to check that out.

A friend actually just sent me a tourism statistics article from https://gowithguide.com/blog/south-korea-tourism-statistics-2023-all-you-need-to-know-5254 as well. It had some really useful overviews on popular regions, things to see/do, seasons to visit, etc. Thinking it could be a big help as I start piecing together my route.

Spam alert??

@Kate thanks for the suggestions! Gyeongju and Seoraksan National Park definitely sound worth checking out. I'll have to add them to the map. And no, I haven't tried KFC breakfast before but it sounds intriguing - might have to give it a go while I'm in Korea just for the experience! Appreciate you weighing in, the trip is really coming together now thanks to all the helpful advice I've received. Looking forward to being on the road next spring!

@Peter yeah, I think so hmmm

Definitely spam alert!