Could I substitute white Miso in place of doenjang?

That sounds like a great idea! Given how very salty miso can be, I'd start by adding a small amount to the peanut butter (unsalted) and tasting a little bit of it, adding more to taste. I'd also use crunchy peanut butter here, for more texture - and I'd use a seeded bread, for a more complex flavor, overall - but of course, use whatever you have, or like! And let us know what you think, please. I'm curious! ;o)

Your taste buds, your call.

After all, that's the most important thing: pleasing the stomachs of those who will eat this.

This is a no brainer. Kimchi, bread, peanut butter, and miso are all affordable commodity products. You can make a very small test sandwich without taking out a second mortgage.

If you like it, then make a bigger sandwich the next time. It's not like you're experimenting with a very expensive whole beef tenderloin.

For what its worth, the Japanese enjoy miso sandwiches.

Best of luck.