Revisions completed. Currrent Patch with this inevitably showing up on SMWDB eventually.
Level is to the right of the Hub on the OW. I have made a couple of minor changes since this patch, however. There's also a full version for Shivering Cinders but that's not important right now.

➡️ Overall readjusted difficulty to be harder, as I feel the previous revisions made the level too easy relative to where it is in the game.
➡️ Redesigned parts in to help provide more fairness. I'm sure I missed a few places that can be improved, just tell me.
➡️ Redesigned Section 1 a little bit to account for the behavior of the current version of the Venus Fire Trap being used. Also changed a bit of the end of this section to be more forgiving.
➡️ Starting part of Section 2 has On/Off blocks to block out the player from trying to do a quick reset of the room if they're running low on air while trying to get the first SMWC Coin.
➡️ Moved a Question Block in Section 4 (contained a flower) to be earlier on and changed it to a Yellow Switch block.
➡️ Both variations of the air meter used in this level drain slower.
➡️ Redesigned the beginning part in Section 3 to have some required swimming while avoiding Cheep-Cheeps and to illustrate that the purple poisonous water drains the air meter faster. Removed the Parakoopa it had no use and a number of issues that don't really feel like going into detail over.
➡️ Changed a Question block (contained a Feather) to have a Flower in it instead, since that would be more useful for this level.
➡️ Redesigned parts in sections 3-4 to have no short-hopping Dolphins (they weren't re-inserted in the base I used anyway). Conveyor belts are mostly used in their place.
➡️ Made much greater use of Fishbones, which only appear in Briar Mire in a non-threatening manner. Their current miscolorization will be accounted for in the (hopefully) next base ROM update.
➡️ Changed level music to 37 (Massive Layoffs), since its updated versions sounds like a better fit here.