An unnecessarily fiddly mechanic for a berserker class…
When you fly into a rage, don’t roll any damage you take until the rage ends. Instead, write down the damage dice (including modifiers and tags like piercing or ignores armor) for later. While your rage lasts, your HP are unchanged and you deal +1 damage for every damage die you have written down.
When your rage ends, roll each of the damage dice you have written down, one at a time, applying armor or similar modifiers to each and taking that much damage. Then erase all the damage dice you have written down.