Funny, this thread bubbled up just now. I’ve been doing a lot of experiments with grease pencil V3 the recent days and you’re right, there is no reasonable way to do this conveniently with the latest stable blender.
But if you can wait just a another few month longer or if you are willing to maybe give the alpha version a try you can sail around a lot of GP’s shortcomings by just using geometry nodes.
It is not yet feature complete though. Not all nodes work and everything is still a bit wonky. But this 
is what you’re after, right?
flat_stroke.blend (2.8 MB)
I used @higgsas edge offset node.
It’s a great node. I use it all the time. Thankyou higgsas 
and as it is a curve to mesh conversion, we are working with standard materials. So procedural stuff is also on the table now. 