Knuffel as donation perk seem like a bad idea to me. With items it's easier to let it go, because there aren't many people who want to collect exactly every and each item on the site. However more people are hung up on collecting all of the knuffs, whether they like them or not. And with the unique knuffel feature and the custom knuffel being open, many knuffel collectors (and there are quite a few) might get frustrated to a bad point, like leaving the site, because they'll feel it's pointless to stay.
I think custom and unique knuffel ought to suffice, for now at least. Maybe in the future knuffel as donation perk would be an option, but right now people are pretty upset about the unique knuffel feature. I think adding a knuff as donation perk would increase the bad atmosphere.
I'm not sure detail is the problem with the perks

Back when the gifts were gold, I still liked and disliked some. Now it's the same, only now we have more color choices. I still really wanted some of the new non-gold gifts after the new system was introduced. So far I've been hoping it would just need some time for the users to get used to the new system.
Maybe it is indeed the color that adds to the feeling of "this is something that belongs to the queen, it's royal, I want it".
Or maybe it's the lack of "fancy" (lol). Right now the perks are not gold and indeed they look like any other item on the site. Previously, seeing a nice item in gold I'd always wonder if it maybe has been a gift of the queen, now there is nothing that differs the perks aside from the name tag. Still, that seems like a minor problem, if it is even the problem.
Another option is, perhaps people liked to see the old items tweaked into something new. Like, we know it already, but it's even nicer, more unique. Following that, Firn, have you considered slightly changing a TT item and listing it among the donation perks? It doesn't have to be in gold necessarily, but maybe if it was slightly altered and in some nice color, perhaps people would donate more likely? See, with a completely new item it's a hit or miss. But if you take an item that you know was popular and add something to it, maybe that would help.
Or just make one or two of the perks in gold and see if that helps?