Welcome to the forum and yup, it is OK to ask DIY stuff here. Whether you get the answers you want is another matter.
I suspect there is a a lot more going on with the joists. It is highly possible that the sagging old ones have been that way for so long they are now permanently bowed. The new joists probably spent better than 50-percent of their stiffness in fighting the old joists to bend back down when you removed the jacks. It may be a game of diminishing returns.
Only the 12' is the correct size for those spans and spacing with 2" x 8". For the 16' you are going to need 2" x 12" before you bolt and glue anything.
I am of two minds in how to approach this given that the original beams may be set. My first thought would be to jack it all up again and sister 2" x 12" to the other side of the stock beams, but again it will be fighting the set.
The preferred approach to my mind would be to use a temporary steel beam and jacks to raise the floor without lifting the old joists. Lever the floor up so the nails come up out of the old joists. Then sister the new 2" x 12" to the old bent beam and remove the jacks and steel beam. Then nail the floor to the new "sister."
Generally, I will not give you the answer to your question, but I *will* guide you into discovering how to solve this yourself.