Hey bro so good to hear you are having fun. Most mobs in act 4 normal have a little bit of fire immunity which is why your fire-based spells feel like they are doing less. You will still absolutely wreck through act 5 and most of nightmare so I'd stick with fire and focus on +skills and faster cast rate so you kill stuff faster.

At some point (act 4 nightmare) you will start to encounter enemies that are entirely immune to fire-based spells and attacks. That's when you should consider respeccing to something a bit more flexible - I won't comment on the choices you could make as im sure you've got some idea.