I will add my 2c to this. Proxmox is effectively a DISTRO (based on Debian 11 currently) whereas LXD is SNAP or a Deb package. So in other words, at the risk of stating the obvious, Proxmox installs from an .iso, so for example to test or use Proxmox, your talking about creating a “Proxmox VM” or “Proxmox server”, whereas with LXD your talking about a vastly more portable and flexible Deb pkg, and/or better yet installable as a SNAP.
The Orabuntu-LXC project will be releasing Orabuntu-LXC v7.0.0 this year (I feel confident we will release our alpha which introduces fully automated LXD cluster support for both Ubuntu Linux 20.04 and Oracle Linux 7 and Oracle Linux 8 in Q3 2021). The support for these platforms for LXD clusters is already available in the master branch at the GitHub.
CentOS, RedHat, and Fedora to be added for LXD cluster automation in Q4 2021 (possibly also by Q3).
Orabuntu-LXC uses the SNAP LXD deployment mode for the support of automated LXD cluster deployment in the currently supported platforms of Oracle Linux, CentOS, RedHat, and Fedora, as well as Ubuntu (of course).
So my main point is LXD is the clear choice for heterogeneous enterprise server environments unless you are ok being tied to a customized distro based on Debian 11.
HTH