Adding more floor joists in a crawlspace. Originals are 5' OC and need more support
What you are calling a floor joist, appears to be much thicker than a 2x floor joist.   Maybe like a 4x8 or something.  So it's probably a "bearer" resting on the piers, which would explain why they are far apart.   Typically you would then rest floor joists across those beams, (or use Simpson hangers if the floor needs to be nearly flush to the beams -- which is your situation).

This is typical ...



This is where you need to be with Simpson joist hangers:



Can you measure the thickness of the wood that you are calling a floor joist (which is really a bearer)?   Also, that beam that is notched badly is not good (most likely).   This would be the time to address that notch, which is fortunately very close to the wall.   Can you measure the depth of the cut on that notch?

An easy way to do that would be to set a pier underneath the bearer so that no notched portion of the bearer is under stress.

Once you have addressed making sure that the bearer is properly supported, you can then hang joists between the two bearers you showed with joist hangers.  Then you would put the subfloor and whatever choice of flooring you want on the joists.