All right, we finished a sort-of WIP (which took way waaaaay too long to make), thanks to Kilaye which helped (read: did 80% of the actual work) making it. It creates an almost-optimized city (we left two 9x9 I blocks out) with some weird choices, which still achieves the 600.000 objective.
It was made on BizHawk 1.1.1b (because that was the version that was available when I started making this... like 8 monthes ago?) there:
http://rayas.shinra.fr/SimCity_final2.bkm
We don't think we'll improve it, and we don't think it's polished enough for TASvideos (played on Easy, we add two rails section in order to spend the money faster because why not, we put at least 2 useless electric pole). Movie ends as soon as the last I section is put in place because you don't need any input to get through taxes and goals menus, they just disappear after some time. Yet, it achieves the goal at around frame 210000.
It's way less ambitious than what FatRatKnight envisioned, still, it manages to get the city done. This game is horrendous to TAS, it was a nightmare. Glad it's done.