I am not sure about bread communities, so if you come across some, please let me know as well!

On mixing flours, it is all about preference, and what you have available to you. I am lucky to have a mill that provides me with barley flour, so I always use a mix of barley, rye, and whole wheat as this adds a subtle complexity to the bread. Using only bread flour or whole wheat flour always makes "dull" bread, in my subjective opinion. So, I never bake only with one flour.

Baker's percentage is a good way to "write" bread recipes. So, easy and hopefully straightforward, most bread recipes are written in the following way:

100% flour;
x % water;
x % sourdough starter;
x % salt.

My go-to recipe is always:

100% flour;
80% water;
20% sourdough starter;
2.3% salt.

If you make 1000g batch, so it is basically 2 loaves of about 900g after baking, the recipe in baker's percentage reads as:

1000g flour;
800g water; (80% x 1000 g flour)
200g sourdough starter; (20% x 1000 g flour)
2.3g salt; (2.3% x 1000 g flour)

I hope that this makes sense. If not, please don't hesitate to ask questions. Maybe you have come across these kinds of recipes.

As for the buckwheat, I have tried it but it is not as available here and can be very expensive. I have tried so many different flours to be honest, from millet to buckwheat, to spelt to linseed and so much more. Depending on how much you add to your normal bread flour, you can make something special!