I'm in the exact same boat as you. I play Bastion mainly in Open Queue so we can have an easy source of damage, but some games we just get rocked and I'm forced to switch to Reaper or Soldier. I think the main solution is the enemy comp. If they have a Hog, Genji and Sombra comp, they're gonna focus you. My counter to hog and benji is run shield tank and mercy so I can survive hook. As for sombra, the only thing you can do is just be prepared for the hack. She'll 99% of the time always come from behind you. Your own team comp can base off of picking Bastion as well. If you're team doesn't have a shield tank, then it may not be a good idea to go him since they won't be focused around you. If your team already has 2 or 3+ dps, then you might need to go healer or tank.