There is a lot of effort to be done if the Dutch doesn't want to deal with
other revolutions once a while.
Though Catholicism was a great cause for the revolt, but the social disparity in both halves were also a great issue: the Lower House of the General-States had the same amount of seats for both the north and the south, despite Belgium being more populous than the Netherlands (fun fact: today it's the opposite) and the North had more seat in the Upper House since it had more provinces. A other important fact is that the Northerns saw the South was a conquest, rather than a unified territory, which was annoying to say the least.
So Willian needs to do a compromise for the union to survive in the long term:
1 - DON'T be an autocratic dickhead.
2 - Reform the States-General to accommodate the demographics, that would mean that a Southerner domination of sorts is likely, but the Northern dominated Upper House can balance the things.
3 - Don't upset the Catholic classes, yeah the King had good intentions in spending public funds in developing the South (like in Antwerp), but he has to cope with the society of his new subjects.
For consequences... I'm not a fan of Paradox-style butterflies, but changes are going to be pretty big: While it wouldn't a Great Power like Prussia or France with the industry of Belgium and the shipbuilding of the North the UKN is going to be a relevant economy in the continent, a important point to consider is that, IIRC, the Netherlands weren't a neutral power like Belgium which is big deal.
For the Germany thing, I don't see Belgium influencing Prussia's downfall and dodging Austria's decline in any way other than "because I say so", with German and Italian nationalism growing the only way to a multinational empire like the Habsburg's to survive is to reform/divide, which is IMO another PoD rather than a consequence. Also, IMO, any French government is going to support Sardinia or any Italian state against Austria, it's like you are a big guy that has another big guy as a rival and a small kid wants to deal with your rival, it's just too good of a chance to waste.
Apology for poor english and grammar, it's probably my largest post there.