Just sharing my settings which I use for EndlessGrowth, and my reasoning balance-wise:

This is all assuming you set your max level in EndlessGrowth settings to 100 (restart game to take effect)

Breakpoint level: 8
In-game description describes level 8 as "employable professional", so anything lower I believe should be taking more damage due to lack of experience. This will allow you to gain a significant upper hand over a group of low level raiders, and rewards your colonists that have seen numerous battles.

Early slope: 0.125
Means that level 0 toughness skill will take x2 damage. Accurate for someone who has 0 experience being attacked, vs the level 8 employable professional who takes regular 1x damage.

Max reduction at level 20: 0.105
Will result in 0.9x damage taken at level 20, which is actually fairly significant when combined with armour and good melee skill. "Legendary" in toughness doesn't mean bulletproof imo, it means that they can just about survive hits that would kill anyone else, and just about bleed slow enough to be saved in time. They survive, similar to how they do in Kenshi.

At level 100 (max), they will take 0.2x damage. Combine this with the vanilla tough trait and robust gene, your looking at 0.075x damage taken, which is broken, but if you grinded that far you deserve it.

Additional mods to make this actually achievable:

"Vanilla skills expanded"/Mad skills: The critical passion from VSE increases learning speed AND disables forgetting for that skill, essential for EndlessGrowth imo. I think Madskills is a little cheaty but that's also an option. Make sure to disable "critical affects passion" in VSE settings.

"Passion on level up plus" is also essential. Allows you a chance to gain a passion when reaching a certain level of skill. Which means, eventually, you will gain critical passion (from VSE) if you level that skill high enough. I recommend enabling "natural config" and setting the forget ratio to 50%, otherwise you risk losing your hard earned toughness skill at high levels.