After buying an external charger yesterday and leaving one of my batteries charging all night I was able to get flasher to boot. Then I flashed Fiasco and installed backup-menu. Then I restored the BM backup I did before I started all this mess. After that I installed cssu-testing then I installed a few very specific packages on top of that (either .debs downloaded from cssu-devel or .debs build locally from cssu-git)

Now I have 3 BM backups that I am currently downloading to my PC for safety (one is from before installing cssu, one is just after installing cssu and the last one is after I installed everything)

And now I have an external battery charger that can charge BL-5Js (and other types of rechargeables including all kinds of Li-Ion packs)