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Can't recommend you any certification, but did you ever use Gentoo or NixOS? You can learn much while using them. As for professional recognition, I think only Red Hat certification programmes are worthy.

Thanks. I have been using ZorinOS desktop and Ubuntu Server. I will look into the Red Hat certification options. I was initiallly thinking something more distro agnostic like Linux +.

I am somewhat biased towards certificates - I see them more as vendor's confirmation of yours' proficiency in using specific software/hardware in a way they expect you to. As for Linux kernel and it's wide and extremely diverse ecosystem, there is no single entity, who are in charge of judging what is OK and what's not. IMO, closest to this position are Red Hat due to their contributions and enterprise services (Red Hat Enterpise Linux distro, it's components they've developed exclusively for it, testing environment like Fedora and CentOS Stream, OpenStack, Ansible...).