This totally works. Totally the same, CR returns to 0 and TN is variable.
Math and formula is the same, but 1 less number calculated by players to reach TN typically. Remains compatible with core except roll module and monster stats. (Welcome to the post stat block world)
Remains compatible with D6 only use; 3D6 ( at-2 base TN) or 4d6 (at + 4 base TN to maintain 20 spread in roll)
Hits can still work off of 6s as opposed to the HP model.
Speed of turn is the same. Possibly faster for players who can’t add.
Easier for ICRPG or D20 players to digest. 1 less variable number for players to add.
edit side note: your damage reduction armor system you are using, player hit TN, was where I think @Shadymutha started with this in September/November. Very Heavy OSR exposure probably got him to the current system.
Removing the 10 from armor would have been a faster route to the same end.
Side note 2
Another idea is TN 5 is default,
Simply justify in your head why you are at your TN thematically (not because it has the character hitting 50% of the time). Express it in your descriptions, then try to be consistent with it.
If you want a terrifying monster, TN 14 with everyone making a Hard Charisma save or all actions are Hard 1/2 effort unless trying to flee works great!
Why TN 14?
It’s really dim light +4, it smells bad+2, it’s really humid +2, sounds travels weird here +3, roots make the terrain a tripping hazard +2…well +5 (default TN) makes it 18…take out the really dim light and you are at 14.
So the place smells like a dead moldy body, humidity has your clothes and armor sticking to you…you feel things crawling on you and hope it’s just sweat, roots and loose stones are all over the ground, and your companions sound like they are in a well 20 feet away when they are right next to you, and more clearly when away from you…then you see it…everyone please make a hard charisma check!
(It can be a longer description, but this post is already too long)