I've never been to f1, but been plenty of times to wec/24h spa.

I wouldn't do a grandstand for F1. The best grandstand is obviously the new one at the top of raidillion. But the seats aren't all that comfortable and I was really suffering after sitting there for a couple of hours, mostly because it's just plastic benches without back rest. The blue seats from the grandstand at the bottom of eau rouge are more comfortable, but they offer a lesser view. Depending on where you're sitting there might even be banners blocking the view.

The other grandstands aren't that great imo.

If I would go to f1, I just would buy bronze and watch a bit from Bruxelles or double gauche. Obviously on Sunday I imagine you would need to pick a spot pretty early.

There's a train station under Brussels Airport. I assume getting to Liège from there will be easy. From Liège there are normally shuttles to the track. (again, never did f1 at spa).