Thanks. I'm debating with myself over whether to dig it up or not in order to run some cat 5e to the garage. IIUC, you can't run ethernet or tv cables in the same conduit as 120V or 240V. It's a toss-up whether to run a cable or just do wireless. If I do re-dig that trench, there will be some 3" conduit going in alongside those two 1". Something in favor of digging a completely new trench just for a new feeder is that it would reduce the distance by 25 feet.
It will be a 4 wire feed to the garage, whether I use the existing conduit or dig it up and put in a 3". I will be installing a ground rod at the garage as onenew32 advised back at the start of the thread. All neutrals in the garage sub-panel will be isolated from the grounds. Found a 20 space Square D 100A panel for less than $150 so I may be rewiring pretty soon.