The fortificaton skill mod is called Fortify Skills. Here is a link https://www.nexusmods.com/valheim/mods/172

The config file allows you to fine tune it, like how far the fortified skill lags behind the regular skill, etc.

It seems even work on other skill-add mods such as Vitality, Ice Magic, and so forth.

be aware that it apparently comes with a default 50% XP gain on all skills which I did not know about until just now. He probably should have kept that vanilla as a default value.