Sadly there is no desktop OS that would be on par with GrapheneOS in terms of security.
When choosing a desktop OS, first think about what you'll be doing on it and how it relates to your threat model.
If an iPad with external keyboard and mouse would work for you as a replacement for desktop OS, that would probably be the most secure option. Obviously not everything can be done on iPad in a convenient manner.
ChromeOS is a good option for security if you're not afraid of Google for privacy. For example, I use a Chromebook in Guest mode for banking and nothing else.
MacOS on Apple silicon macbook is a fair option with good balance of security and convenience out of the box.
Windows had made significant improvements in terms of security in recent years. If you get Enterprise or Education edition you can further improve security and disable unwanted telemetry via group policies but it requires some effort.
QubesOS is nice for an easy way to manage many VMs but you need to understand its security limitations.