I think the VYOS team should come out with a robust TCP MSS clamping recommendation.
After reading the above comments and also the VYOS knowledgebase on TCP MSS clamping for PPPOE connections: https://support.vyos.io/en/kb/articles/pppoe-sub-interfaces-2#comments
I see that both of these point the user to setting the TCP MSS clamping on the LAN interface, whereas the actual clamping should take place on the WAN interface.
There is another option referred to by a Ubiquity thread: https://community.ui.com/questions/TCP-MTU-MSS-values-for-PPPoE/6218d940-a69e-40a3-93ba-12d5ab8c3455
and this uses the firewall option to set the TCP MSS clamping. It also turns out that VYOS also has this option and using this method allows the user to set the TCP MSS clamping on the pppoe interface itself (WAN).
set firewall options interface pppoe0 adjust-mss 1452
On a side note it is also possible to assign a policy route to the pppoe0 interface itself, but this does not give the correct clamping in VYOS.