You have to understand that, under the hood, Chex Quest is just a reskin of Doom. The code changes are minor and anecdotic (zombie-replacement monsters don't have ranged attacks and don't drop ammo when they die, episodes end after level 5 instead of after level 8; that's about it). Digital Café wasn't really a programming studio, and they had a tight deadline, so they didn't change much in the code.
So then in ZDoom, the Chex Quest actors are just directly derived from a Doom actor, with sometimes a cosmetic change. For example, look at [url=https://zdoom.org/wiki/Classes:FlemoidusBipedicus]this one[/url]. It's just the shotgun guy, with drop removed and ranged attack reduced to melee range.
Everything works like that. It uses the same sprite names as the shotgun guy, it uses the same sound names as the shotgun guy, and it largely uses the same actor logic as the shotgun guy. If you load a mod that contains a full replacement of shotgun guys, it's easy to make it also replace the biped flemoid.
Now Strife, on the other hand, was made by a real game studio, and they were very ambitious. They took a game engine developed for a shooter, and they made it into an action/RPG game with inventory management, stat upgrades, conversations with NPCs, item shops, persistent levels you can revisit, etc. It's not just a Doom reskin, it's really a completely different game, just based on the same engine. You don't have a 1-to-1 mapping between Doom monsters and Strife monsters; or between Doom weapons and Strife weapons. While a Doom mod can be turned into a Chex mod with a minimum of effort, a Doom mod just cannot
be changed into a Strife mod without a lot of hard work.
It's also important to work that Strife has a very steep power progression for the player character. With stat upgrades, every weapon becomes more powerful and useful (the dagger can even end up causing a lot of damage once you have enough stamina upgrades), and weapons such as the grenade launcher, mauler, and of course Sigil are extremely powerful. Incendiary grenades in particular are really overkill, much stronger than even the sigil. The BFG is a toddler's toy in comparison. Game balance adapted for Doom is therefore completely unadapted for Strife. So not only is porting a Doom mod to Strife difficult technically, but it's not even desirable because you'll have to change everything to rebalance.
Don't forget that the Mauler's secondary mode can one-shot Inquisitors. I have Shudaki of Halved Studios' let's play of the DOS version to thank for that. (Running on Vavoom or Zandronum I think)
I see what you mean regarding the main game. I guess it wouldn't be as simple as inserting the Brutal Doom code into a copy of the Strife VE source code and writing original code for the Strife exclusive assets. That is if one can call coding "simple". The talismans can also boost your Hidden Blade power significantly in the re release.
I'd love to see a Brutal Strife once Sgt Mark IV has the chance to do one, maybe even a Brutal Marathon as well if he expands his knowledge to Aleph One too.
Brutal Pathways into Darkness may be a little tricky as that game currently remains Mac only and the source code for it hasn't been released, unlike Marathon.