Can I remove 2 GFCI breakers and install a tandem if the box accepts a tandem?
I don't see any GFCI's. Your cheapest route is to replace the blue handled 15A shed breaker with a BR1515 and add the wires from the unmamrked 15A circuit on the left to the additional termination you just added to the shed space. That's a lot cheaper than buying any quadplex.