It's not a hack. It's literally having the tunnel at 1500 MTU.

Check my answer to the sibling comment [0]. It's also known as mini jumbo frames, and is documented in RFC4638 [1]. And here's a post [2] talking about using it on OpenReach FTTC, which is similar to my own infrastructure, only I'm FTTP.

[0] https://news.ycombinator.com/item?id=48589677

[1] https://datatracker.ietf.org/doc/html/rfc4638

[2] https://blah.cloud/networks/enabling-mini-jumbo-frames-rfc46...