3 weeks in Asia

Hi! My fiancé and I are trying to plan a trip. We were given 3 weeks time off for it, but we are struggling to decide where to go. We have considered Vietnam, Thailand, Korea, Japan. This would be our first time in Asia. In an ideal scenario, we would visit all these places in one span in order to save money on flights since we will be visiting from Texas. But thats unrealistic, and i do think that we will get soooo tired of traveling constantly, and overwhelmed trying to see everything. Honestly we’re not great at following plans we set for ourselves either, we are wanderers and like to go with the flow when traveling.

The most important thing for me, is seeing things I couldn’t see in the US, so the prettiest beaches possible, the wildlife (huge for me), trying new food, night markets, river markets etc (also huge) and Architecture and historical monuments/temples would be last priority, although we would like to see some.
For my fiancé, he is really interested in Japan, going to the colder parts of japan and Onsen towns as well as staying in Tokyo or some bigger city and getting to experience what it’s like there. (He’s really mostly excited for the convenience stores there lol.)

If anyone has any recommendations, let me know what you think we should do, where we should go, or What you would have done differently! Thanks!