Thanks @marc_s, virtualisation is a good point, the proxmox host MTU is default (=1500), so that likely explains my max 1492 PPPoE MTU.
However, is there anything more to gain than 8 bytes/0.5% per packet? E.g. non-TCP/UDP traffic (Wireguard comes to mind, which seems to be working on with 1492 MTU)?
If I want to get 1500 MTU in PPPoE I need to edit the proxmox host interfaces + the underlying VLAN interface + the adopted VM MTU, the chance I’ll forget one of those is significant.
The advantage of having pppoe mtu at 1500, is preventing running into mtu problems…like you did.
Only TCP is fixable with mss-clamp, all other protocols rely on Path MTU Discovery, which requires proper ICMP allow rules along the entire path between client and server