You may try /usr/share/.. firewall4/templates/zone-mssfix.uc and ucode/fw4.uc from github.com/openwrt/firewall4 master + firewall4/templates/ruleset.uc from https://github.com/openwrt/firewall4/pull/22
Those adjust MTU for offload and shorten default slowpath, so you can have soft offload for all uses without any weird wifi freezes. What you see currently is finally corrected pppoe offload in kernel.