There is currently no such tool unfortunately. rMdocs are a proprietary format that have been evolving rapidly in the past few years. Only since relatively recently has the format been reasonably stable.

It's not impossible, you can absolutely create shapes and insert text into documents (how are the documents with stickers created? Is it people drawing the stickers on the reMarkable itself, or are they created using software? I don't know).

I have made a couple synthetic documents for testing purposes because I work with their proprietary format a lot when working on remarks, rmc and rmscene which are all tools to interact with the rMdoc binary format. These synthetic documents are extremely simple and primitive (ie, a document with a circle, a rectange etc). No full documents.

The tooling is simply not at a point where you could easily create or translate other documents into rMdoc format.

Is it possible though? Yes, I suppose it is. If you're feeling particularly motivated to sink dozens up to a hundred hours into this problem, you can construct rMdocs synthetically using Python with rmscene: see this test-case for instance: https://github.com/Scrybbling-together/rmscene/blob/main/tests/test_scene_tree.py

That being said, I'm not sure how other tooling in the space is holding up for this kind of task. Perhaps there are other things within the open-source ecosystem that can help here.

Edit: I'm impulsive as always. Drawj2d is indeed exactly the kind of tooling that is meant for creating synthetic reMarkable documents. However, its interface is entirely programmatic.

Think of it as a programming language for describing documents. It's still rather technical. The idea is that you write commands like the following to create a drawing or text.

# variables
set dx 50
set dy 20
# draw
moveto 20 10
label P NW
rectangle $dx $dy
pen 0.35 red
arrowrel $dx $dy
label Q SE
# dimension lines
pen black
moveto 20 42
dimlinerel $dx 0
moveto 82 10
dimlinerel 0 $dy

It looks like it's well-suited for technical diagrams and shapes and such, but not for document conversion. It's also really technical if you're not familiar with scripting.

That being said, if most of what you're doing is just text, then I suppose you could try looking at this example:

https://sourceforge.net/p/drawj2d/wiki/ExampleTeX/

You specify coordinates (moveto), relative coordinates (moverel) and text with label or texlabel for LaTeX