Affair of the Box for Mourning Candles is definitely one of the best grinds. War of Assassins for Collections of Curiosities is even better but as it relies on the Tomb-Colonist card it’s only worth keeping a few Collections on hand at any given time. The Fidgeting Writer is best done in large batches to reduce potential losses from variation and also less clicking around in general. There was a thread a month or two ago suggesting using a Gang of Hoodlums and the new Topsy King option of one action for 1 CP casing to go on a heist at the law offices with one Inside Information. According to the post the 5-progress reward is consistently better than the 7-progress reward even if you have enough, as the lower one has the chance of giving a Searing Enigma instead. (Plus heists give some MW as a bonus.) Spirifers can grind Souls through Unfinished Business and trade them in for an EPA slightly above the Affair of the Box, while it’s of course difficult to compare to the Fidgeting Writer. With high watchful the Gallery of Serpents is very profitable as with certain choices it gives the (always fate-only) unpredictable treasure alongside Uncanny Incanabula and sometimes bonus supplies. Since they’re the same fate cost it’s not worth grinding the Temple of Uttermost Wind if you have high enough watchful for the Gallery.
I just bought my first Overgoat today after many months slowly grinding alongside stories and seasonal content. I have the Goat Farmer’s Browser Extension installed so can see the total echoes I have available at a given time; since I was getting pretty close already I went through all my items and lowered the number I had reserved of most, so was able to find enough extra cash for the last stretch. Even if an Impossible Theorem is super cool, it’s pretty useless and costly to make so there’s not much point in having ~50 spare enigmas hanging around.