If you DON'T want to (or for some reason can't) use a mod to delay the appearance of the expansion content then it's still fairly simple to avoid it: don't take the boat to Solstheim, and don't take the teleport to Mournhold. You will get attacked by an assassin sometime during your first few rests as a result of the Tribunal expansion, but as soon as you talk to a guard about the issue, the assassination attempts will stop and nothing more will happen until you visit Mournhold.
(Just don't use the assassin's armor, since it's Tribual-quality.)
OTOH, aside from that armor, most of the unbalanced things either require a fortune in cash (relative to a starting character) or fairly high skills/survivability to obtain, which won't be an issue early on. The few things you can do in Mournhold that won't get you killed at low level don't pay much, and aside from a couple of talk-to-people missions and a single escort (which likewise don't pay much), you'll get splattered in Solstheim long before you reach anything powerful.