I used to purchase notebooks for journaling but had a similar hangup to some other posts on this thread. I just though that it was a hard sell to continually have to shell out a bunch of cash for the sexy notebooks when all the materials for a passable alternative are essentially free if you can tap into the waste of a typical pre-pandemic office space with a comercial printer. (For a while there was a thing that would happen with the network printer where it would suddenly begin to print gibberish, usually only one or two lines per page, uncontrollably for reams and reams of paper. This was the source of my first roll your own notebook pages.)
At some point I heard about the Midori system and then realized that if you had a reusable Traveler's notebook you could print the style of paper that you wanted to use, fold it, and have an A5 sized folio (?) insert that you could staple with a specialized stapler to make the paper inserts.
This is what i have been doing for ~3 years now.
https://papersizes.io/
A5 Travelers Notebook: https://a.co/d/iy32n37
https://print-graph-paper.com
Swing-Arm Swivel Stapler: https://a.co/d/0afXOaW