Had similar struggles as well. Then after few experiments with plain paper (tried A4, B6, A5 - all landscape), I switched to Midori Paper Pad, carrying it in a leather pouch. Using A5 on the go and A4 on my desk. All in landscape orientation, like slides. My output is mostly diagrams/schemas and tables, very little prose.
Advantages so far: high quality per that works with fountain pen. No fear of ruining whole notebook with an ugly note or a drawing. I can carry only sheets that are relevant to the problem I am solving right now. I can lay out the sheets in front of me to get a bigger picture. I can throw away the bad ones. It is easier to scan (using iPhone scan to PDF function).
Can confirm, MUJI notebooks are of a decent quality and they can handle fountain pens as well.
In addition to that, I gradually switched to their 0.38 pens of different colors. With fountain pen it was more stylish but impractical for multiple semantic(*) colours. With cheap, light, yet nice quality pens of different colours I can not only carry multiple ones, but have the same set at multiple places.
(Dark blue is body text/drawing, light blue is comment/secondary annotation, orange is action/process/message passing, green is data/metadata flow, …)
In Czechia and Slovakia we have mapy.cz which gives you more detailed map for walking/hiking, including contour lines. See [^1] for example.
The coloured trails you can see at more detailed zoom levels, are part of the hiking trail system. The outdoor signage even has a national technical norm [^2] (in Slovak, but has some terminology translated int to English and German).
Advantages so far: high quality per that works with fountain pen. No fear of ruining whole notebook with an ugly note or a drawing. I can carry only sheets that are relevant to the problem I am solving right now. I can lay out the sheets in front of me to get a bigger picture. I can throw away the bad ones. It is easier to scan (using iPhone scan to PDF function).
https://md.midori-japan.co.jp/en/products/mdpaper-pad/