Any non-trivial complexity codebase eventually implements a mediocre SQL/Lisp/etc.

Exactly! Why take a system not designed for this sort of scale and force it to scale, rather than use systems which are designed and tested for this scale and volume? All you will do is hackily re-invent all the other things that the other databases had to do to scale to this extent.

Plus size is only one limit, you would be limited to 1 write every few milliseconds. My napkin maths estimate is that there are at least 1-2m writes per hour going into this thing, so probably 300-600 writes / second (Average) and maybe over 1k writes/second peak. We are going to fall over here!

Not sure why some people seem to have a viwe of "There is no scaling problem that can't be solved with a sufficient enough number of SQLite databases".