And they didn't adopt Christianity. Now, that makes them the underdog, and it makes them a target for anyone who wants to Christianize them.
That's an interesting point you made here : what about an earlier christianisation not driven by Frankish rulers?
OTL, Anglo-Saxons monks already tried to do such, mainly at the call of Austrasians, but failed. Maybe a christianised Frisia (that would be another point to be discussed) would be an incitative?
Of course, the main issue is germanic peoples usually converted or by conquest, or because the population they ruled was mainly christian (with christian elites present) or thanks to "benevolent influence" of powerful neighbours.
But a saxon leader, powerful enough to submit some of his neighbour could see an interest to christianisation.
As you said, political unity (rather than centralisation) was hard to achieve outside old roman lands, it was generally hugely personal (like Samo's Kingdom) and fragmented again after the death of the unifier.
But let's Wilfrid manages to convert Aldgisl of the Frisian Kingdom, wouldn't that help christianisation of Saxony, and therefore, the religious help to a unifing ruler?
So I'll give it a shot: if you want to prevent the Frankish conquest, your first and foremost need to divide the Franks. The Frankish kings divided their lands amongst their sons, so killing off one of their kings at an opportune moment (i.e. kill one off when he has a bunch of adult sons, ensuring the Frankish realm is split up into multiple pieces) seems a good POD.
Unfortunatly, I see some things that can't happen here.
1) It wasn't more the kingdom that was divided than the public land (of course, in Frankish mentality, public land and kingdom were the same). Franks considered the divisions as each heir recieving a fair share and not living the familial cell.
2) Multiple pieces ask to be nuanced : the divisions roughly followed the same lines (romans, geographical) except for the south that was more considered as a foeign land (it's why Aquitaine was divided equally among Clovis' heir rather than given to just one).
3) This idea led to regular re-unification. The death of a king or a majordomo was the opportunity for another king/majordomo to take over his land.
Clothar, Dagobert, Pepin II, Charles Martel, Peppin III, Charlemagne...You have many examples.
Even without reunification, you still have to deal with Austrasia that was the senior Frankish kingdom : most rich, most populated, most powerful...And direct neighbor of Saxons.
Paradoxically, what you need is maybe a slighter more powerful Austrasia and Neustria.
A stronger Neustria means Austrasia can't take over the west of Francia and could turn earlier against Frisia and Saxony.
It could be an incitative for Saxon rulers, that could do better as they won't have to deal against Franks+Aquitains+Lombards+...
If you add that an earlier christianisation...It could help, but even if united, even if christianised...They don't have the number to hold a full scale invasion.