IIRC the mirror is from a spy satellite, but the chassis and electronics are all NASA.

This is true. The telescope is much more than just the primary mirror.

Roman has two science instruments, a wide-field imager (“WFI”) and a coronagraph (“CGI”) and they were both built by NASA for this purpose (optics, electronics, detectors, deformable mirrors).

The WFI optics includes filters and a prism and grism for spectroscopy, and of course the very large CCD arrays.

The coronagraph is a rather special tech demo and allows imaging of exoplanets (Jupiter-like) while blocking the light of the host star. (https://science.nasa.gov/mission/roman-space-telescope/coron...). It’s a precursor to HWO which is mentioned down-thread.

From what I can gather it is much more than just the primary mirror but the entire platform sans classified parts, which were replaced with NASA's optics and sensors.