I had the opposite experience in research. I switched from pencil to pen. Reading later (e.g. 15 page math derivations) I could see some of the dead ends I took and mistakes made, while still following the thread.

Additionally, some research results should be stored in an immutable material.

try a fountain pen with erasable blue ink

I had no idea that erasable ink (for fountain pens) is a thing. That’s awesome!

You're going to need this: https://en.wikipedia.org/wiki/Ink_eraser

(We called it 'ink killer' in school, you were however not allowed to use it the first few years after switching to ink from pencil.)

addendum: And you can get ballpens with the ink as well.

I highly disliked them, the result always looked off to me. At some point in school, I reverted to just striking through everything.

Yes, and they smell like pee

Erasable blue ink tends to fade away after a few month/years if the paper is left a bit exposed to air or humidity, since the eraser uses a chemical that is also present in the atmosphere. So be careful if you intend to use it.

Whats even more awesome: The ink is not erased, but literally made invisible by manipulating the molecues in the ink "responsible for the color". By reverting this chemical process, the ink can be made visible again!

Invented by Pelikan in weimar germany!

I too still use pencil and paper

I have an absolutely huge project folder split into hundreds of topics, and a master ToDo in Excel used as a kind of Kanban/white board, with development notes in Word.

It's clumsy AF - the Excel part works, the Word part could be better - but it's a working solution of sorts, with everything synced across all my devices so I can make some notes on my Macbook while out and then pick up the thread at my desk later.

I tried all the usual alternatives - Trello, Org Mode, paper notebooks, real white boards, and so on - and so far this is the most productive.

take that UML