While I hate the idea of ads, IMO it seems like the safest way to keep the place running long term.
You could always do the OP's mentioned options of only having them on the front page or only for guests etc, but if it earns too much perhaps donate the excess to charity once you get enough of a buffer saved up? Could do a pool of charities and rotate through them whenever the bank balance gets big enough, could have people nominate and vote on the charities even for that extra transparency factor.
The donated servers seems the nicest idea theoretically, but I figure there's be hardly any people that can both afford that and are generous enough to do so indefinitely, and what happens if suddenly a few of them pull the pin at the same time due to another GFC sort of thing? (maybe there's already been more than enough offers in the comments to cover it? I saw one skimming the first page but didn't read it all, would be awesome if true)
If going the old skool donations route, you'd need some idiot proof mud map guide for n00bs on how to get teh crypto and donate it - US companies locking you out of easy donation options like PayPal, Patreon etc really blows for making this easy, but it is understandable given what this place is.
The affiliate sponsorship option sounds the most sketchy for long term stability - even if you get a company to do it, they get one nasty lawyer's letter & it could be all gone again.
just my worthless 2c