So I got to the bottom of this today. Swapped mcu, pmic and ram chips between my working n3ds LL and dead one - working was still working and dead was still dead. Desoldered emmc from working console - it started to show bootrom error. Desoldered emmc from dead one - still 0,09a and nothing on screens. Also, just for testing purposes, tried too boot working console from ntrboot - it doesn't boot and doesn't show a bootrom error, just black screens and blue light, power consumption is 0,11a (and yes, I did everything correctly, checked multiple times on my working o3ds). Anyway, next I swapped SoC chips between this boards and board from a dead console started to show bootrom error! Soldered to this board emmc chip from working board and it now boots fine, except upper screen doesn't work, only backlight, but I think this is not a related issue and I'll fix it later, didn't have enough time today.
Tl;dr: the problem was dead SoC.
Will post photos of SoC swapping process and bootrom error next post (for proof).
Huge thanks to
@Sono and evryone in this thread who helped me with this repair attempt.