I thoroughly enjoyed this map, and it's not a thing that happens often.
This is a really good emulation of grim's style. Void dissolves props, the colored lights are still there, brushwork is as crazy as ever. Even the sounds are closely reminiscent of the trilogy. Not only it is close to the original, but you also have managed to improve on that particular style massively.
However there are certain things that are missing from the formula.
- The first one is healing items. This map is being incredibly stingy with health, meanwhile the original maps gave you a considerable amount. When/if working on the next installment, consider aiming to give player just enough goods for player to complete the map on the first try if they play carefully but not overly defensive. Try using item_item_crate with dynamic resupply so player[s] get what they currently need.
- The second thing is a little obscure but grim actually hid small secrets around in maps, like brush signature hiding behind displacement or secret room in I think the second map in the series. This map is completely lackluster in this regard, which is a bit sad.
- Original maps had more puzzles and various things you would interact or otherwise engage with, and this map is more of a walking simulator.

Other general things to consider:
- You use ambient_generics as a room tone sounds - don't do that, create your own soundscapes for that. Not only the current approach will break in multiplayer (no, I don't care that you don't like the multiplayer, suck it up), triggers in Source very rarely can ignore player leaving their bounds, leaving the sound playing. Making your own soundscape may seem like a hassle, but it is highly worth it, since it allows you to practically sculpt the sounds of an area exactly to your liking.
- Try to use the soundscapes in more granular fashion. Oftentimes you have a soundscape reserved for a big room playing in upcoming smaller rooms, which is not that great.
- You may want to clip the stairs. Some people experience bouts of nausea from the rapid camera ratcheting.

Overall, I like this map a lot. Very thorough attention to detail, I loved little touches like localized gravity in the rotated room.
This abstract style is a breath of fresh air during the onslaught of unoriginal backrooms maps and other similar trash.
I'd love to see a follow-up soon but I have enough experience with hammer to know that it won't be the case. Good luck and godspeed!

Thanks a lot for this very detailed and incredibly helpful feedack, this is exactly what I been looking for, I very appreciate it.