I definitely agree that reroll is a hard decision. The game's limits, at least for the earlier parts of the game, really don't work well with a reroll mechanic. The best approach might just be abilities or shop items granting rerolls. For a guaranteed reroll, maybe if you skip a shop with 20 faith? That's a high cost, not easily achievable, but also exactly when you'd want  a reroll. There's also always the option of banishing units to make the game a tad more reliable in the lategame.

As for supplicant, my personal opinion is that it'd work perfectly fine at just +1 faith.  Whether you gain faith at round start or a unit's death makes little difference. The former is a bit more reliable and can be doubled up on but the latter actually combos with more options. That said, it might also be more unique to keep Supplicant on 2, and on death combos feel like they need that bit extra eco anyway. Alternative nerfs might be dropping Supplicant to 1 Faith but it gains a second Faith if another unit dies before or after it. Increasing the cost to 4 would go against the cost design but break the aspect of immediately going to 6 Faith. One really unintuitive nerf would be "buffing" it to 4hp since it'd be less likely to die, although that might also cause frustrations with players since it cant die round 1.