This mod is made for those who don't want plow through enemies later in the game. You could of course simply choose not to invest in perks with damage bonus but to me, the game doesn't feel as enjoyable when I deliberately have to avoid using certain features. You'd be missing out on the satisfying grind to unlock them.

I suggest using this mod together with other mods like Unbogus Health Scaling or Immersive Scaling that prevent the player and NPCs from gaining more health by leveling up. If someone wants a version with even weaker perks let me know and I'll put one up.

Here are the specific changes:

Basher
Rank 1: 25% --> 15%
Rank 2: 50% --> 30%
Rank 3: 75% --> 45%
Rank 4: 100% --> 60%

Big leagues
Rank 1: 20% --> 10%
Rank 2: 40% --> 20%
Rank 3: 60% --> 30%
Rank 4: 80% --> 40%
Rank 5: 100% --> 50%

Chemist (duration)
Rank 1: 50% --> 25%
Rank 2: 100% --> 50%
Rank 3: 150% --> 75%
Rank 4: 200% --> 100%

Commando
Rank 1: 20% --> 10%
Rank 2: 40% --> 20%
Rank 3: 60% --> 30%
Rank 4: 80% --> 40%
Rank 5: 100% --> 50%

Demolition expert
Rank 1: 25% --> 15%
Rank 2: 50% --> 30%
Rank 3: 75% --> 45%
Rank 4: 100% --> 60%

Gunslinger
Rank 1: 20% --> 10%
Rank 2: 40% --> 20%
Rank 3: 60% --> 30%
Rank 4: 80% --> 40%
Rank 5: 100% --> 50%
*Decreased range bonus as well: 25%, 50% --> 10%, 20%*

Heavy gunner
Rank 1: 20% --> 10%
Rank 2: 40% --> 20%
Rank 3: 60% --> 30%
Rank 4: 80% --> 40%
Rank 5: 100% --> 50%

Ironfist
Rank 1: 20% --> 10%
Rank 2: 40% --> 20%
Rank 3: 60% --> 30%
Rank 4: 80% --> 40%
Rank 5: 100% --> 50%

Rifleman
Rank 1: 20% --> 10%
Rank 2: 40% --> 20%
Rank 3: 60% --> 30%
Rank 4: 80% --> 40%
Rank 5: 100% --> 50%

Rooted
Rank 1: 25% --> 15%
Rank 2: 50% --> 30%

I was going to nerf the perks even further but any lower than this and nobody would probably want to invest in them.