I checked my old thread here and yes when I did this on the hg612 I did leave it alone so didnt need to increase the mtu on it, it was in bridge mode.
https://forum.kitz.co.uk/index.php?topic=12842.0As far as openwrt is concerned tho there is multiple interfaces as has been stated.
You have the individual ethernet interfaces and also the switch interface. Both would need bumping I think. I think someone already suggested this to you.
The bridge interface is likely to be named br-lan.
Also on my link I had to do vlan2 also, which was an extra virtual interface assigned to the WAN connection after pppoe was established, its possible openwrt does something similar. This extra interface doesnt appear until after you connect so is something you change after internet connectivity is established.
I think its eth0.1 you need to set to MTU 1508 based on your ifconfig output. After thats done try setting the pppoe interface again to 1500.
So try this in order as I am not convinced the bridge needs any adjustment since the lan bridge is not part of the wan connection..
eth0.1 to 1508 then pppoe-wan to 1500
if that fails then set eth0 also to 1508 and try again after pppoe-wan to 1500.