please add an option to change the thickness of something written with the pen

I see your point. If it can thinner or bigger, I can use it easily. So, I hope that it can upgrade.
https://happywheelsonline.io

A slider to choose the width, like in Samsung Notes, would also be great

This is a must! I have been used to writing with my 0.7 pencil for 25 years. My current notes look to me as if someone else wrote them. Please make this a priority or I will have to go back to my physical notebook because this way they are not saved in my memory.

Custom pen sizes - not just the 3 settings. These custom settings should be saved as pen profiles for the user - for reusability.

My main struggle at the moment is not being able to have the right thickness. A sliding bar would be nice, like the one on MS Whiteboard.

There are only three pen size available for writing and none of them is useful for writing. Like literally the smallest size is too thin and the medium one is too thick. I suggest you give a slidable option to adjust the thickness of pen just like the iPad version. And pls add more color and give us option to select and make our own color.

It would be great if I could click on the stroke button twice and adjust line width by popped up slider.

mimicking the iOS implementation of this would be ideal!

I noticed the same thing. Maybe a sliding scale of thickness would be best, then you could find the perfect size for you.