This are my absolutelly favorite and long term use writing/thinking tools: pencil, paper, eraser.

I've only found two places that sell a wide range of mechanical pencil 0.5mm leads. There's probably more.

4H
3H
2H
H
F
HB
B
2B
3B
4B

(HB and B are best)

Pentel: https://www.pentel.com/products/super-hi-polymer-lead-0-5mm-12-pieces-per-tube-box-of-12-tubes

Jetpens: https://www.jetpens.com/Uni-Smudge-Proof-Lead-0.5-mm-HB-Aqua-Case/pd/44708

This is my #1 choice pencil: Alvin DRAF/TEC retractable. I bought a small case of them 15 years ago and still have unopened boxes. Only one of them has actually ever failed. https://salestoresusa.com/alvin-dr05-draf-tec-retrac-mechanical-pencil-0-5-mm-retractable-tip-ergonomic-grip/ They were once a lot less money. What else is new.

I don't actually use this stuff, just covet it: too expensive. Mostly I use Bienfang 10 x 10 gridded bond. But Clearprint still makes 10 x 10 gridded pads in vellum. https://www.gsdirect.net/products/clearprint-1000h-16-lb-10x10-fade-out-grid-vellum Lovely shit, vellum.

And this is the perfect eraser: Staedler-Mars white. Also ideal for cleaning printed-circuit board gold plated card edge connectors. Good general cleaner for small gadgety stuff. Very mild abrasive does not harm/remove the gold: https://www.staedtler.com/intl/en/products/pencils-and-accessories/erasers/mars-plastic-526-50-eraser-in-premium-quality-526-50/