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How much little pipe slope you need depends upon your plumbing code.

The IRC plumbing provisions require only 1/8" drop per foot of pipe run for pies that are 3" or greater in diameter. This amounts to only 1 1/8" drop over 9 feet of run. It is likely you can keep that slope. UPC may be 1/4" drop per foot in which case you would drop 2 1/4" in the same 9' run.

Boring the 2x12 joists will be a bigger problem.

Although a 3" PVC waste pipe can be bored through 2x12s and stay within prescribed load limits (4" pipe is too large), you also must maintain the bored hole at least 2" away from the top and bottom edges of the joists at all times.

So trying to place a 3" waste pipe through 2x12s can still be a problem.

The common solution would be to cut out about 12" of the joists and add headers/girders to the ends of each side. You will create a channel between them that will allow the pipe to be installed without any boring at all.

You would have to lower the Tee connector at the soil stack to assure proper slope if it is too high.