You can do PVP Frontlines, it's a great way to cap the tomes and how i cap mine like 1 Year ago when i was raising PVP rank, you can also do Trials, but those barely give any Tomes, you can also do Palace of the Dead, also nice way to cap tomes.
If people only do the Two new Dungeons and suffer from burnout when capping the tomes it's their own fault, there's more ways to cap it, i don't see why only do the Two Dungeons.