Here's another idea. GF wants to go to Nan too. So maybe KC too far. 3 week trip. The rangers told me Khao Yai is best in Dec or Jan as elephants come out of bushes more when not hot. Be in January. So Khao Yai sounds good. Phimai is easy stop. Then hunt up to Kamphaeng Phet and onto Nan the back via Kho Kor.
Adding a beach adds a lot of km