Let me break down the facts in this thread and what you can derive from them.
Fact 1/ The Huawei HG659b gets great (900Mbit?) speed on Spark UFB (using PPPoE).
Derive: The Spark network is fully capable of providing maximum speed for the Fibre Max plan.
Fact 2/ The Netgear Orbi gets great speeds using IPoE with 2D.
Derive: The 2D network is fully capable of providing maximum speed for the plan. The IPoE stack in the Netgear Orbi works great.
Fact 3/ The Netgear Orbi gets pretty mediocre speeds using PPPoE with Spark.
Derive: The PPPoE stack on Netgear Orbi is underperforming. What specifically it causing the problem is only something that Netgear would know.
Unfortunately this is a very common theme with Netgear routers. This is the 3rd different model of "Gigabit" Netgear router that I personally know of that has this issue. It *was* even an issue for the Netgear router that Spark sells at it's stores. I say "was" because Spark raised that fault with Netgear and they released a version of the firmware for that router that now has a PPP stack that works great. But sadly they haven't done for others in their line-up. Yes Spark has raised the issue of the general theme we are seeing here with them. I would suggest you log a fault with Netgear and hope that it doesn't fall on deaf ears.