These are issues related to snappymail more so than cyberpanel admin panel itself. As for os of choice Ubuntu has the least issues and bugs than CentOs. You can post your email logs for the ubuntu server here so that we can help you.
There is no such thing. A provider like hostinger can offer centos with Cyberp preinstalled but doesnt make it the ideal combination. That is just their business model. CentOs is free/open source and doesnt come with cyberpanel its caos os from the old gravity os. Its just like any other rhel linux distribution.
I would recommend almalinux if your are hellbound on using only rhel based oses. But for the longest time I have setup and managed servers i choose ubuntu 20 anyday.