If you are not yet using IPsec with IPv6, you can, for now, disable all those rules in the filter list with IPsec in the comment, that includes the IKE, AH, ESP rules, and the HIP too. The fewer rules enabled the better, because unlike with IPv4, fasttrack is not supported with IPv6 so we would want to reduce CPU resource spent on firewall processing (more rules more CPU load) if possible.
Also, I don’t know if it applies to Vodafone. But many ISPs now support RFC 4638, which means an MTU of 1500 is supported for a PPPoE connection, which means no more headache and workaround for MTU problems like you experienced. My internet connection is over PPPoE too and it works with MTU=1500 without any issue.
You can try to see if it’s supported by first editing the MTU of the underlying ethernet Port (for instance ether1) to 1508. Then in the setting of your pppoe-out1 interface (which you named Vodafone) set both Max MTU and Max MRU to 1500. If you redial and see Actual MTU has a value of 1500, then RFC 4638 is supported by Vodafone and you can remove the previous fix with the MTU field in ND.
You can then go to the same speedguide.net/analyzer.php page to check whether MTU is now really 1500.