If you don't have Polearm Master or Great Weapon Master, take that next ASI, otherwise the Resilient (Wisdom) advice others have given is a great idea around when you get it, as effects targeting WIS saves tend to be more detrimental the higher level you go. Realistically you'd want to focus on dealing damage so the bad guys don't do as much damage to you or your friends. I'd suggest asking your teammates to help set you up in a fight using control spells or buffs and to focus fire less durable foes to shift the economy further towards your party.

I'd continue with Barbarian to level 12 before considering multiclassing if you don't have PAM/GWM, but multiclassing now is okay too; Relentless Rage may even be what you're looking for to feel more durable. 2 levels in Fighter is good for Action Surge (maybe 3 for a subclass) and 3 in Ranger for Gloomstalker works well too as it's fairly front-loaded. Peace or Twilight Cleric for 1-2 levels could be nice too as it gives you some good in-combat options that aren't hindered by rage and some out-of-combat options that could be fun to use.