How can I add a ground wire to my older Square D panel?

This is a Square D 225 amp subpanel, manufactured in 1978. It is located in an old home that predates electricity (originally had knob and tube), and as you can see it has a lot of issues. The grounds and neutrals are connected, the panel is fed by a three wire cable, and most of the circuits are ungrounded, all problems I am eventually going to fix.

However, I would like some advice about what I should do to correct these problems. First of all, can someone familiar with this model identify where the bonding screw is, or if it has been removed? This panel is located at least 50 feet from where the power comes in, and it would be a pain to try to fish a ground cable through the conduit that feeds the panel, and so I was thinking of running a new separate conduit with a ground cable in it.

I also plan to ground the ungrounded circuits by running a single-strand green wire to the fixtures on the circuits.

Is there any problem here with this plan, and are there any other issues with this panel?

Note that while this is a 225 amp panel, it is fed by I think a 2/0 cable, and is one of four subpanels in this house (I haven't checked the others yet, but I suspect they are also wired improperly).

Along with the supply cable there are, I think, 7 cirucits whose breakers are located in the main panel where the power comes in, and pass right though this subpanel in the same conduit as the feeder wires, but I think that their neutrals connect in this subpanel. I would like to change that to where the breakers are in this subpanel. This panel has had these issues for more than 40 years, but an electric water heater was recently connected to it, so that concerns me even more.