Version v21 has (easier) im too young to die which is basically the original game with the new enhancements, whats the stance on using this difficulty?
From what I know, ITYTD in BD v21 is the same as Wussy in BD v19 which is the most runned version of the game (source: https://www.speedrun.com/brutal_doom/thread/7dxqs ).
I haven't tried speedrunning BD in v21 so it's possible that I'm wrong here but I don't think there is any kind of a major difference between those two versions. You should be fine with running v21 on ITYTD.
Yeah, theres another one though, literally called ITYTD (easier) which levels stuff out, cuz its insane how much damage you take
I'm fairly positive it isn't allowed to submit runs on that difficulty but you can probably train/practice on this difficulty then jump on the appropriate one to set a time.
What do you want to run atm? Monsters damage output isn't that high (except melee) imo but it's true that IWADs runs all have really tough beginning but it gets easier afterwards, so it's fine to die a lot.
You can always try playing NoMo to learn how to navigate some levels properly as less time spent on a level = less damage taken. You can also train Ultimate Doom with only E1-E3. Those are nowhere near as tough as E4.