I've ran the Sharpcap's sensor analysis on my Canon EOS Rp (modified). It costed me 500+ shutter actuations. I know some people here on CN use this camera for astrophotography. This analysis should help deciding which ISO value is best for your particular needs. I've attached the resulting table and made a plot of the read noise and the full well values.
As you can see, indeed the noise is very high at low ISO. Only at and after ISO 640 the noise becomes manageable. The full well values look good comparing to recent dedicated astronomy cameras. The dynamic range is so-so (around 11).
In my opinion for the EOS RP camera the ISO 1600 is a good compromise shooting DSO objects. 640 may also be good.
Oh, and Sharpcap is a really nice piece of software! I also analyzed my ZWO ASI183mc and the values closely match the values from the ZWO website.