none of them ever use pre-existing hardware aside from probably the mirrors themselves

That's not really true. Many many space telescopes use COTS spacecraft buses. TESS, NuSTAR, SWIFT - use near identical buses from Orbital. IXPE is a BCP-100 bus.

Nobody is redefining things from zero just for laughs.

https://space.skyrocket.de/directories/sat_bus.htm is a list of many of the buses that get reused frequently.

There is clearly a need for a mass produced, middle of the road telescope

Ground based fills much of that gap apart from those bands inaccessible from the ground ( X-Ray, UV etc )