Well yes I suppose work out the recording time you'd need and go from there.
The Alexa Mini has but one card slot & I've never had a cfast card fail from a Canon camera, so one card slot isn't as risky as the Youtubers would have you believe.
Also unless you want to spend 2X on cards, you're unlikely to ever do dual recording anyway!
In regards to C100/C300 having superior 1080p, I suggest you test them against the newer cameras to see for yourself. The newer chips have better DR and colour for sure.
Even though I loved the C300II, I would hardly call it future proof as its almost EOL. I mean it'll make nice pictures for years, but on a technology front it's on its way out. I sold mine earlier this year because I believe the CRL is lowkey the best development Canon have made for a while... hence why its in the new C500II.