In short: another way to handle this is to create a brush shape that only covers half the shape area, so it doesn't extend past where you place the stylus tip.
To explain the logic of it: digital brushes are typically made with small transparent images, by default centered in a designated box. Some programs have multiple ways these images can be applied, but in Rebelle, all brushes "stamp" that shape (or shapes) many many times along a brush stroke, generally creating an unbroken line. (You can increase the spacing in the "stroke" tab of the brush creator if you want to, say, easily make a dotted or dashed line.)
Tablet styluses have a small point, enabling us to see fairly clearly where we are touching the screen, for the sake of precision. It imitates using pens and pencils fairly well, but it can result in a bit of visual/logical disconnect when used for larger things (like a wide paintbrush), because the marks made can expand well past the stylus tip in all directions. But even when using larger brush sizes, the lines or marks made stay centered on the tip of the stylus, due to the shape itself (usually) being centered. That way, if a large brush tapers smaller with pressure or the like, the thinner section of the line stays where you'd expect it to be relative to the stylus. If the shape were asymmetrical, or placed off-center, it could end up offset from the stylus and feel "wrong." But, in this case, that's exactly what you want.
Now, for the detailed how-to: since you want your tortillon to
not expand past the stylus tip, you'll want to create an asymmetrical brush shape that only fills in half the shape box. (Generally speaking, you'll want to use the right half, though you can rotate or flip the shape within the brush creator if you need to.) You can either create a square canvas and use grid lines to note where the halfway point is, or draw on whatever size canvas you want and use the selection tool once you're satisfied with your shape.
For a quick test to make the visual example below (zoomed in at 200% for clarity, because I was working with a small pencil), I just mucked about with my mouse on a regular sized canvas. I scribbled a triangle so the left edge would be flat, then made a square selection on an empty section of the canvas, so I could transform it without changing the shape I drew. (Transform mode creates a small "+" in the center to help with orientation, which you'll want to line up with the left edge of your shape.) Then, on a duplicated brush (so I wouldn't overwrite one I like), I opened the brush creator to the shape & grain tab, clicked the shape box, and used the "import selection" function.
(This will be a faster method while you figure out what sort of shape you want. Once you've found one you like, I recommend saving the selection as a png and using the "import texture" function. Then, if something ever happens and you lose your custom shapes within Rebelle, you still have the shape on hand to re-import. Also, once you're done adjusting your new brush to your satisfaction, or at least get it to a decent state, click the "save changes as default" button. That way, if you make a change you don't like, you can just click "reset brush changes" to get back to a decent state, without having to start over or try to remember what all you adjusted. Also also, if there's an existing brush shape you otherwise like and want to edit, you can make that brush large and click once (or a few times, if it's too transparent) to stamp that brush on the canvas. Then either erase the left of it to create a flatter edge, or create a selection so that its left edge is in the center.)
You can see the test shape I made on the right, and how it looks imported to fill in just the right half of the shape box of the brush creator UI. On the left, I made a vertical line to test with, set my tortillon to blend, and scribbled with my mouse to see how it worked in sorta-practice. For a direct comparison of how it avoids the issue normal brushes have, I made the two horizontal lines below the scribble. The top line is the pencil I was drawing with set to blend, and the bottom line is my tortillon also on blend. (It's less opaque because the many many "stamped" shapes aren't overlapping as much as the pencil's hexagon, since the latter fills the entire shape box and my triangle is relatively small. This can be adjusted in the stroke tab, via the spacing and opacity settings, if your tortillon isn't as strong as you'd like.) For both lines, I placed the small brush cursor shape(/crosshair/tip of the stylus) right on the left edge of the line I drew, then clicked and dragged right. As you can see, the normal pencil created a bump past the line (as you have observed with normal brushes), and the tortillon did not disrupt the line at all.
Note: the left edge of the brush shape doesn't have to be wide and flat. It can be rounded, or come to a point, or whatever works best for you. It can stay the same width horizontally, or taper to be wider or thinner on the far right; stay solid throughout, or fade to transparent at the far side or edges; be clean, or textured, or even have a few little scattered dots to imitate loose graphite, etc. (Texture can also be added by changing the grain, located next to the shape. Loose graphite dots can be added (and thus removed) separately by making some dots as another shape, then adding that as a second shape to your brush by using the "+" below the shape box.) Play around with different things to see what you like, just make sure not to leave any markings in the left half of your selection/canvas when you import or save the shape.
Also, thanks for asking about this! I think I'm gonna keep this brush on hand and pull out my tablet later to play around a bit more, and see what adjustments work best for me. I hadn't thought to create a blender like this, and it seems pretty neat with the little bit of testing I did to make this example!
(Sorry, this got a bit long. I apparently ended up in infodumping mode, haha.)
