Meursault is op. Being immune to environmental sanity effects is very strong and just makes farming nightmare fuel harder. Consider reduced sanity effects from monsters as an alternative instead of nullification if you are adamant on this mechanic. Even just immunity to non-monster sanity effects like the evil flowers or something.

Meursault's arms guards are also OP in their current state. 80 base attack power is just nasty, plus being able to upgrade it further to the point where you can one-shot hounds is nastier. Consider giving it 40 base power, which is still high for a weapon you spawn with, but I think the fact that it's a punch balances it out. 80 should be the maximum damage with all upgrades; maybe 90 if upgrades become harder to get.

I don't believe Meursault needs the base attack multiplier either. Being able to use the piggyback, marble armor, chop, mine, and gather faster is strong enough. Plus, he spawns with his gauntlets anyway.

Since Meursault spawns with a portable crockpot and has access to chefs tools, I think it would be a good idea to make it so that eating uncooked food deals minor sanity damage, maybe 5. It would be really easy to avoid anyway, so even 10 would be fine, but I don't want to push it. With meursaults inherent sanity effect immunities, it would be perfectly fine and even of ultility because you dont have to slam down a bunch of greencaps to spawn sanity monters. Ideally, this aspect is added along with the prior sanity balance suggestions.

Enkephalin needs some work too. It drops too often, and the stats are very fine. Consider nerfing the drop rate by 10-20% in certain health ranges. For example, ≥ HP 100 has a 10% chance of dropping one, ≥ 500 HP have a 60% chance of dropping 1, and ≥ HP 700 has an 80% chance of dropping 1. The listed 1000 hp range is fine enough as is, but for higher hp amounts, the quantity dropped should be lessened; for a big kill, the guaranteed drop is balanced. Also consider nerfing the stats for consuming enkephalin. 30 hunger and 5 hp for how often it drops? Either add heavy sanity effects like -20 or -30 or nerf the hunger to maybe 5 or 10.

Golden boughs are strong but fine enough for the difficulty in getting them. Just nerf certain upgrades and consider requiring the use of maybe an enkephalin module or two to upgrade things as well. I cannot attest to the usefulness of using it as a light, as I haven't done anything with it in that aspect, but light is easy enough to come by as is, so I'm sure it's fine.