CAS aren't going to suspend a ban. I was inspired to re-check CAS AC Milan v UEFA to see CAS's options
https://www.tas-cas.org/fileadmin/user_upload/Award_Final_5808.pdf. They are unlikely to send it back to the AC even if they decide (which I doubt) that the punishment was disproportionate.
Para 158 states CAS has the power to
"issue a new decision which replaces the decision challenged or annul the decision and refer the case back to [the CFCB]”.
In AC Milan, it referred the case back to the CFCB Adjudicatory Chamber saying
"In doing so the Panel respects the autonomy of the CFCB Adjudicatory Chamber to find a proportionate response based on a careful and thorough assessment of the underlying facts as provided for in Article 28 of the Procedural Rules."
Critically this was largely because
AC Milan specifically requested that the case be sent back to the CFCB.
It seems very unlikely City would do that. So I expect CAS to decide.
Interestingly, in AC Milan we see UEFA conclusively state in respect of the 2014 settlement that (para 79(g)): "Manchester City [was similar to AC Milan in respect of] the
huge size of the breach in relation to the break-even requirement, ...On a procedural level the Respondent confirmed that in ...Manchester City ...there had been two hearings before the CFCB Investigatory Chamber." AC Milan state (and it is accepted), City's breach in 2014 was even higher than €121m. This is important because it supports the Conn details of what UEFA already knew in 2014.