Took some liberty with the double overline above CCCL. It was suggested on a site, but couldn't find any examples. A single overline (like over the DCCCLX indicates *1000), so a double indicates *1,000,000 (*1000*1000).
Had flashbacks to Catholic School there and a sudden urge to watch Blues Brothers....
I had to dig around and yours was creative, but I don’t really think they had a way of expressing such huge numbers. I never saw the double bar multiplier.
I think a Roman would have said “numerus maximus, ultra descriptionum” (a humungous number so big you can’t describe it).
Sister Agnes would be ashamed of both of us.