Poison is not elemental damage. That is why it is not an option. Elemental damage types are fire, cold, lightning and thunder. There are homebrew feats that enhance poison damage in the tabletop. But it is intended design that poison is resisted by a lot of creatures.
Acid and Thunder are not elements either but you can pick them.
Just because Wotc and Larian both made such a glaring mistake doesn't make them right.
Why just one out of all draconic options for sorcs cannot bypass resistance?
Also looking at it now it doesn't seem elemental adept works on immunity, only resistance in which case I wouldn't ever bother with it. Less damage is ok, your dragon choice undoes that. Full immunity just makes your dragon choice irrelevant.
Ofc I could just respec to fire anytime and spam fireball but thats boring and as it is I don't even need any buffs to fireball and wall of fire.
So far I have 3 specific items larian made for poison builds - robe, gloves and cloak. They clearly wanted players to be able to make such a build, so why must just about every other mob be completely immune to it?
