you can use any custom cert chain you want (including a wild card cert you obtained however you want from Let's Encrypt or other ACME-based CAs):
https://pve.proxmox.com/pve-docs/pve-admin-guide.html#sysadmin_certs_upload_custom
the corresponding CLI command (e.g., if you want to integrate it directly into your ACME client): "pvenode cert set ..." (you need to reload the pveproxy systemd service as well afterwards to use the new certificate), it's also available over the API.