I started a quite nice mod and the more I progress, the more I think that maybe it would be interesting to split it into a rich animation framework (an ESM) that others could interface with, and an ESP (the mod itself for end-users). The problem is that I alraady have a good chunk of things already in place, many things that draw my attention (I am an experienced dev but a noob at TES modding), and as a result I do not want to bother to learn TESVSnip and others for now since I already have plenty of things to deal with.

So I would like a simple answer: will I be able in the future, through third-party tools, to split my mod if I continue to work on it for now as a single unified file ? Without having to reproduce the work done so far ? I guess the exact procedure will involve two copies, transform one into an ESM and the other into an ESP, then flag the ESP as being depdendent of the ESM (is it even possible for an existing esp file ?), then strip every one of those two files from the unwanted elements.

As long as your current esp doesn't depend on other (non-Bethesda) masters, you could simply create a new, blank esm, and then merge just those things you want in it into it.

It's good to hear Ingenue, thank you very much.
Will I also be able to make my existing ESP dependent of this ESM ?

When you check the esp's records into the esm, the CK moves those records out of the esp into the esm, and makes the esp dependent on the esm. Really simple.

Ok ! Thanks again, Inegenue, this is a great relief, really.

Wrye Smash has the ability to "esmify" esp's. I'll be needing to do this to my mod before release as it adds custom worldspace (which is broken in non-esm's). For the time being though, the mod is in development as an esp file.
Not only will this allow my mod to be a little more modular (less hassle updating) it will allow end user's to customise or tweak their experience without compromising any additional packages I'll be releasing.

As far as I know the esp > esm feature of Wrye Smash is completely safe, besides it's pretty much a necessary tool of anyone who wants to mod their game seriously.

It's good to know, Anteras, thank you. While I do not like to use WyreBash to manage the mods I install, I will give it another look for the useful features it may provide to modders, such as the one you mentioned.

Happy to try help out.

I used to be adverse to using it (when you had to install additional python stuff) but the benefits were just too good to pass up. Running something like FCOM for Oblivion would pretty much be impossible without it. The good news is that the latest versions are standalone (simply d/l and install) and rebuilding your bashed patch is much faster.

I say this because I'm sure there are other utilities out there but Wrye has other pretty useful features.