DM had us roll and create in a vacuum before meeting the other party members

Have very rarely ever seen this go well, party composition and reason for adventuring are usually quite vital things and leads to exactly the situation you've outlined which can hugely impact player enjoyment.

If they aren't willing to change and you're stuck as you are, you could go down different routes.
- Build up a friendly rivalry (as characters) like, trying to outdo each other
- Specialise is something slightly different, e.g. if they're all damage, go tanky with a shield and more defensive build
- Depending on your class, the sub classes alone can offer completely different levels of utility
- Personality being completely different can make you stand out the most, make slightly different decisions of how to interact with party members and NPCs, not every meat-head is the same!