I haven’t tried the transparent mask before,
My method, for making a “crisp” transparency, has been closing off my outter edges of my sketch, Then use a selection tool, to select my blank area, invert the selected area and shrink it by a few pixels, then, create a layer under the sketch, to do a fill in, then a layer between the fill layer, and sketch to do colouring. That usually eliminates a lot of the worry for the white edge, when you shriink the selection in some, though cleaning that white fill background still might be needed after you add in colours.
here is probably a much simpler method, but it is what i am used to now hehe.
As for sketching first, then scanning etc, that works, but I’ve gotten to the point of loving my digital work much better now, just because, I can zoom in, I can create layers and delete them if I’ve messed up without losing my original sketch(es) Some of my projects before I commit it to a single image, has 20+ layers at times, just so I don’t accidentally ruin my project.
My early digital work, one piece in particular, I really liked how it turned out, but, I painted it on the actual white background. Not on a layer. I later had a request as someone wanted it on a mug. Placing it on a white mug was no problem, however, the request was for it on a mug with a black base.
I think I spent more time, cutting and erasing around that imagee to touch it up, that I had originally spent painting it.
I am a big fan of using layers for htis reason now.
And while, you may think now, " no worries I am not planning on needing a transparency" It is best to plan ahead. krita save files, are great for that, they will preserve yoru layers, and when you are happy with the result, you can export to png or jpeg etc… as needed.
Also, a layer is handy for if you need to revisit the project in any way later.
I do calendars, that are themed about a pup, its parents, the farm animals and wildlife surrounding the area. There are items in the sketches I will want the same, ( like the farm house , some certain destinct trees, etc ) and if these are saved as layers, I can import that layer into a new art piece as needed, to adjust, or recreate, where that is needed.
One problem with sketching and scanning, is those pieces, are not a transparent asset that can be re-used.
I use a drawing monitor ( huion kamvas 22" ) and love it, though, my power connection is developing a short which is worrisome.
I’m considering now, a standalone art device once I can afford that.
I would also, watch the krita tutorials, specially on brush making. While you may or maynot be ready, the right tool, can mean everything to your project.