I have completed the base game and the first expansion (Ashes of Malmouth, which ended too abruptly in my opinion). In any case, I’m looking for advice on what I should do next. Should I work on my rep with the existing factions from the base game and the first expansion, or should I move on to the next expansion? Any suggestions would be greatly appreciated.
Welcome to the forum. 
It’s up to you really. If you think you’re ready for Elite difficulty then you can move on to that. If you find it too hard you can always go back to Normal and do the Forgotten Gods expansion.
One thing you could do is leap into the start of Forgotten Gods and do the first couple of quests there which will allow you to ally with one of the Witch Gods factions and get a movement rune if you’re interested in those.
Then you can go back to Elite difficulty or carry on with the FG content.