I'm prepping to take my campaign into the Anarchy era. It's a lot more "Sanboxish" than the Uther era, which I know my players will enjoy. To help me prepare for this, one of the things I'm trying to do is organize the various Saxon Kingdoms to identify their various motivations, and create a "countdown clock" of sorts, so that I list what happens if the player's don't get involved. What I'm hoping to end up with is a scenario where the players are playing "whack-a-mole" and have to rush from threat to threat slowing one of them down, but watching the others get worse because they were absent. If they focus all of their attention on stopping Sussex, the "Countown events" for Anglia or Cornwall will proceed unhindered for example.

While doing this I stumbled across a question about King Ælle. Assuming things go "according to the script", during Anarchy Ælle successfully gets tribute from his neighbors (most notably Duke Ulfius) and allies with Cerdic of Wessex. Ælle is focused on becoming Bretwalda, so he has many of his resources tied up squabbling with his rivals Aesc of Kent and Cwichelm of Anglia. Ultimately, he and Cerdic defeat Nanteleod at the Battle of the Wetwood (formerly Netley Marsh) in 508.

The next time we see Ælle named specifically in the GPC is at Badon Hill. Apparently he has been declared Bretwalda and appears to have united the Saxons. Doing a quick scan, I can't seem to figure out exactly when he is named Bretwalda. It seems it would most likely be immediately after the Battle of the Wetwood because the GPC says Arthur is willing to consider help from even the Bretwalda in 510 before the Battle of Bedegaine. Alternately it could be in 517 after the "Northern Saxons" (like Deira) are defeated by Arthur and retreat to Sussex.

If Ælle was recognized as Bretwalda in 508 after the Battle of the Wetwood, why don't we see him being more active between 510 and 516 (when the GPC says he raids "most of southeast Logres?"). I understand that he is likely only recognized as Bretwalda by Sussex, Wessex, Essex, and possibly Anglia and that the "North Saxons" like Deira may not recognize him as their King, but I don't see evidence Ælle is involved up north. With the British fighting a Civil War and the army tied up fighting the Saxons in the north, wouldn't that be the perfect opportunity for Ælle to strike in the south?

Perhaps the "North Saxons" are his rivals, and he was content to wait to make his move until both they and Arthur had weakened each other through years of fighting? So with this theory, he's calling himself "Bretwalda" and is recognized by Sussex/Wessex (and Essex/Anglia?) after 508 but isn't recognized in the North, so he just consolidates his power and raids, but doesn't make a move until after his rivals in the North join him?

Any other ideas on what Ælle might have been up to during the Boy King era? Did I miss something?

I have been considering that too, although only quite briefly so far. My take on it is pretty close to yours; Aelle is letting his enemies/rivals fight one another first. As a rough timeline:

508 Wessex & Port submit to Aelle to get Sussex warriors against Nanteleod. After the Saxon victory, Aelle starts pressuring Kent, Essex and Anglia to recognize him as the Bretwalda. (Note that this differs from Saxons! where Aelle declares himself Bretwalda only in 517.)
509 Aelle invades Kent and forces them to submit to him, before their allies from Essex can help. This makes him the de facto Bretwalda, since Anglia + Essex can't match his resources to take him out, and Deira and Nohaut are not interested in sending their forces south.
510 - 513 Aelle expects that the Cymric civil war will rage on for a generation; once the Boy King is dead, the other Kings will start fighting one another. The longer it lasts the better it is for him, so no reason to go poking in there and remind them of the Saxon threat. Instead, Aelle spends him time beating Anglia and Essex into submission, but Deira and Nohaut refuse their allegiance still.
514 - 515 Either Aelle is still busy with Anglia, or he is adopting a wait-and-see attitude towards the new High King. Northern Saxons still don't follow Aelle.
516 Arthur is diverted to the North due to a new influx of Saxons there. Aelle lifts a toast to Wotan, as his enemies and rivals continue fighting one another.
517 After the disastrous Battle of Lincoln, the Northern Saxons seek help from Aelle, accepting him as Bretwalda. Aelle launches raids on Logres, gearing up for the big clash between Cymri and Saxons.
518 Battle of Badon where Aelle dies.