We had a not totally dissimilar problem with a room in our house.
The contractor we hired sistered and nogged the existing joists, so they created a new floor level (slightly proud of the old joists) that was flat and level.
Because every joist was doubled up, the floor was much stronger and then a new subfloor and glue down hardwood added. Because you have a long span, this may not be sufficient to address the sag.
esoteric_stuff t1_j1h2cfe wrote
Reply to Options for leveling floor with existing glued down Engineered Hardwood by OllieBrooks
We had a not totally dissimilar problem with a room in our house.
The contractor we hired sistered and nogged the existing joists, so they created a new floor level (slightly proud of the old joists) that was flat and level.
This video is similar to what we had done. https://youtu.be/5-Rvv06_A0k
Because every joist was doubled up, the floor was much stronger and then a new subfloor and glue down hardwood added. Because you have a long span, this may not be sufficient to address the sag.