IIRC the CGI was rendered at a resolution of 1280x768, or 1K @ 5:3.
No they weren’t. The CGI was rendered and printed at 2K, after which it was optically composted onto live-action shots. It’s possible that for some shots they did an optical reduction for compositing which would increase the actual resolution of the CGI beyond 2K on the master negatives.
The effects would have looked horrible at 1280x768. There’s a sweet-spot for resolution and detail when it comes to CGI VFX being integrated to live-action. You want the detail on the final print to roughly match what you get from film. They also have to be photochemically colour-timed just like regular film so that the shots integrate more seamlessly.
Depending on the complexity of the shots they took up to 12 hours per frame to render in 1991/1992. There’s about 4 minutes of VFX and it took about a year to render all up.