Had L5S1 fusion in april, and now have Sacroiliac Dysfunction with failed injections...have severe ddd, should I just fuse the sac joints asap?

Fuse. That would be an option but would encourage you to discuss this with your physician to see it there are any other medications that can be taken for chronic back pains. Surgery is never the final solution as there are cases where the condition actually gets worst with a procedure.
SI joint fusion. Absolutely not! Do not rush into another operation and esp. not that surgery if you did NOT respond to the SI joint injections... Most people have no true SI pathology that deserves surgery, despite what companies make you believe on the internet. Often, there is a problem with the original operation, as you should be gradually feeling better. Maybe the fusion failed, and there is loose hardware.