Correct MTU for Freedom Internet (NL)

I did a quick and dirty test with htop and CPU 0 reaches max 98.6% on download and 100% on upload while running the test, so it could be that. What are the NSS builds? I guess you mean these: Ipq806x NSS build (Netgear R7800 / TP-Link C2600 / Linksys EA8500)

But they don't mention that PPPoE is off-loaded to NSS cores? (only NAT)

Already did, see above

They do and ISP confirmed that it should reach gigabit (although he wasn't an OpenWRT expert, but the hardware alone is listed as compliant in their list)