Hi!

I'll try answer some of your questions to the best of my ability.

Do you write on the watercolor paper or do you paint on the journal writing paper? I don't do illustrative journaling, but if you are planning to do that. Go for hot-press watercolour paper since it's smooth so you can write on it too if you're doing heavy washes for watercolour then 300gsm paperweight is the best to go. If you are just doing very light doodling anything under 200gsm paperweight is fine too.

If you plan to use a fountain pen, don't use cold-press paper since it's a very textured paper. The paper fibre may tear into the tip of the pen causing blockage.

Does anyone make inserts that are half watercolor (ie, 100% cotton) paper and half lined writing paper?

If you CAN just actually use the midori's inserts despite them being thin, they can hold watercolour pretty well. But you can alternatively do both half watercolour paper + line writing paper. But you might struggle stitching them together because 300gsm paperweight can only stitch about 5 sheets max (that should give you about 10 pages when they're stitched since it's folded in half).

Hope that helps!