I run nested Docker in unprivileged LXC on Proxmox on ZFS since 2019 without problems. Gitlab, Nextcloud, mailcow-dockerized etc., for 10 people, - altogether 10 LXC with about 25 Docker services, 1-2% CPU utilization on average, thanks to sharing all resources of the host. I've written a blog post about it [1].

[1]: https://du.nkel.dev/blog/2021-03-25_proxmox_docker/