Glad to see you delving into the world of mapping! Here's a demo of your map, recorded in DSDA Doom (-complevel 9) on UV. My personal recommendation is to watch the demo before you even read my written comments, that way you can get an idea for whether a player is approaching your map in an "intended" way. Otherwise, you may risk me just trying to shove too much of my personal doom tastes onto your map. Remember, I'm sharing only my opinion and experience, nothing more.
I'm no expert on the subject, but I suspect you are a Sunder fan based on the combo of 1. the hex-based cave aesthetic like the beginning of Sunder MAP14, and 2. the large monster counts in small-ish rooms. Your slaughter efforts were valiant, though I fear they fell prey to some pitfalls (though maybe they player exactly as you hoped, who knows).
Quick note on the pre-yellow-door stretch before I talk about the "main event" fights for the other two keys. I think you successfully kept ammo balance very tight for the initial segment, though this does result in sluggish gameplay, as I need to completely line up every shell and rocket to get the most out of my supplies. Tight-fisted sections like this can keep a quicker pace with slightly larger monster counts and a little more leeway with either ammo or health, keeping the same vibe but encouraging the player to be more aggressive. Be aware that the first archvile seems to be stuck on the stairs.
Regarding the yellow key fight: my gameplay video sums up how that fight went. I can run circles around the revenant cloud all day long without ever being in danger. If you want to reduce the possibility of circle-strafing the encounter to death, I'd suggest changing the arena shape so that the player can't easily make a circuit around the horde. A classic way to do this in slaughter maps is go for a "horseshoe" shaped room, forcing the player to double back on their path while also culling the revenant pool to avoid getting smothered. You can also make the initial segment of the fight more dangerous by not starting the monsters already in the center, making the room immediately perfect for running in circles.
Regarding the blue key fight: I think you fixed the issue plaguing the first room, at least initially. There's a lot more chaos here, as there's always a chance of a cyberdemon not behaving the way you want, or you could misjudge a gap in the monster horde and get surrounded. However, I found overall that there was plenty of maneuvering room with little danger, though again, watch my demo to see if the room played out how you hoped. If you wanted to add some spice to this room, you could add a bend to it so that it's possible to break line of sight, rather than just using a big rectangle. You can also experiment with more height levels in an arena, forcing the player to exploit cliffs and drop-offs to manage the horde.
About map visuals: I like your commitment to the hex theme, though I fear your map geometry is too reliant on squares and rectangles to get a lot of zing out of just this one visual feature. Two things I think could help: 1. try more room shapes with some added height variation, and 2. try adding a few more visual motifs to your palette; hexes are good, but it's hard to keep them fresh the whole map.
If you love slaughter, I fully encourage you to hone your craft in this niche. When posting your WAD's in the future, it can be worth posting a few extra pieces of info, such as: gameplay type (probably slaughter in this case), expected playtime, and screenshots (people love these). I hope to see more mapping from you in the future!