Remarkable

Great ideas, all of them. I hope reMarkable keeps their eye on this subreddit.

FWIW, you can do custom templates with a little savvy. The catch is that each OS update wipes out your custom templates and template.json file. I've put in an enhancement request to preserve these changes and even offered an architecture based on our own company's method of preserving "overrides", but no response yet. Technically, you are correct. According to the RM docs, custom templates are not support. There's a difference between not supported and not available. :)

I've also got an enhancement request in to provide shape libraries (e.g. for bullet journaling.) I've got a collection of icons that I use and manually copy from page to page. What I asked for is a managable panel of icons that can pop out from the tool tray so it's a tap & drop (and perhaps scale) rather than so much copy/paste of my icon bar and each icon.

You can turn drawings into SVGs. Check out the Send by mail feature. You can send as PDF, PNG, or SVG. I've used this to sketch a character, then mail the SVG, import it into my 3D CAD package and send it to my 3D printer within minutes. Rapid prototyping at its best.