Yes, you can put tandems on the left and free up 2poles. But...You are asking to add 6 poles. Add a sub panel now and avoid the reoccurring problem you are having with running out of space.
Of course you need to verify you have capacity remaining in this panel to feed a sub first.
I think Cobbler said it best. I put in a 60ft 3/4 loop in the rafters with (3) 1/2 drops and hose reel in the ceiling. All copper. I put a ball valve for each drop. 30x36 shop, I have about $300 in material.
I was just trying to get something in now to use, I can easily add to it in the future.