Honeymoon advice – Japan, Korea, Vietnam or another option

Hi, my wife and I are planning around 3 weeks of honeymoon in March/April.

We are living in a European country. Manila and Boracay is already fixed as one of our destinations as my wife is from Philippines and we will meet her family as well. We will spend around 1.5 weeks there.

That leaves us with a full 8-10 days to explore a new country. The few conditions include:

1) ideally it should be on approximately it's way from Manila to Europe. However, we are also willing to venture a bit further if we find an otherwise perfect option.

2) Out of ~8 days, we are looking to spend a few days sightseeing and touristy stuff and experiencing new culture and the rest in a very relaxed resort in the nature, either in mountains or near forest. Beach might also work but less preferable as we will be in Boracay already.

Does anyone have recommendations based on our requirements?

We are a bit confused between Korea and Japan and also considering Vietnam as a side option.

And help is highly appreciated and will be paid back with best wishes 🎉