I would personally put CloudStack in the same category as OpenStack.
Really the only difference is monolithic vs micro services. Both scale
quite well regardless, just different designs. People can weigh the pros
and cons of either to figure out what suites them. CloudStack is
certainly an easier thing to wrap your head around initially.

Otherwise closest FOSS thing i have found (spent over a year evaluating)
is CloudStack. I am still hopeful someone will step up to maintain oVirt
(Oracle?), but it's clear to me at this point it will need rebased onto
Fedora or something else to keep it's feature set fully alive.