Somewhere out there, there’s an answer for every question! In this thread, there’s very likely answers for at least some questions, so long as they’re quick and related to Fallen London.
Is the most efficient way to grind brilliant souls still converting them from other 3rd tier items, and what would be an optimal grind for a character near the end game?
Probably depends on how fast you can get those other 3rd tier items. Particular heists in Flit are (for me) around 1.26 epa. With conversions …
Let’s look at and 3rd-tier item $item N conversions away from souls. You have a source of those at X epa, and then spend N actions to convert 50 $items to 51 brilliant souls (and get (N-1)/2 echoes of "waste product" that will eventually round up into more brilliant souls along the way). That’s 50/(2X)+N actions to get 51/2 (plus a little bit, but not more than (N-1)/2) echoes of brilliant souls. Solve for X so that the result is 1.26, and you get the lower bound at which it is better to convert.
The 3rd tier chain for increasing N (from 1) here is: mods, gossip, motec, vots, corr plaque, joi, w-s scrap, swa, zz, mol, comprodoc, tot
The X results are (slightly above) [1.32, 1.39, 1.47, 1.57, 1.67, 1.79, 1.93, 2.09, 2.28, 2.51, 2.80, 3.15, 3.61]
So, if you have to farm them, there’s a chance conversions are better for MoDS, inc. gossip, MotEC, VotS. I don’t think you can get correspondence plaques at 1.67 epa or better, and the rest spirals out of control. I do believe getting incendiary gossip or visions of surface at the necessary-for-profit levels is possible if you completed feducci’s wedding.
If you don’t have to farm them, and know that you will naturally replenish any spent items before you will need them for something else, then the calculation is completely different, and way in favor of 3rd-tier conversions all the way. (you would need to do more than 20 conversions to drop below particular thefts effectiveness).