I know this is a few days old, but hopefully this helps. I'm trying to get a hang on modding and this was a useful and relatively easy test case.
I was just looking into something different and came across the method for defining difficulty settings. I'm completely new to all of this and I don't yet know how to make a proper patch, but here is a hack for setting the player damage reduction to whatever value you want. I've tested the damage multiplier it at 0.01 and it seems to work (basically making the character invincible at that extreme). The main problem with this method is that you'll probably have to do it every time there is an update. Here's how you can do it.
1. Download dnSpy here:
https://github.com/0xd4d/dnSpy/releases
2. Extract and open dnSpy
3. File -> Open -> C:\Program Files (x86)\Steam\steamapps\common\Mount & Blade II Bannerlord\bin\Win64_Shipping_Client\Taleworlds.CampaignSystem.dll
4. Find the same file in your file manager and back it up.
5. In dnSpy, navigate to Taleworlds.CampaignSystem.Sandbox.GameComponents.DefaultDifficultyModel, selecting it.
6. You'll see a list of functions. The second one down is GetDamageToPlayerMultiplier().
7. Right click on that function and select Edit Method.
8. Change line 14 to whatever you want the damage to the player to be multiplied by and click "Compile"
9. File -> Save, overwrite the old .dll
That should be it. Make sure your difficulty is on "Very Easy", of course.