I think a paid membership can be interesting from the forum's perspective because the threshold for small often (monhtly) recurring payments is lower then a l less often (yearly) recurrring bigger donation. From a user's perspective donating something for something that is free to access already is not much of an incentive, only if you are really active or involved in the forum this would make real sense.
However as a newbie (correct me if i'm wrong) I noticed this forum is quite big and quite active, maybe there is something more you can do with a subscription. What would be really cool for example would be a subscription where you yearly get a 'multitool.org exclusive multitool' send to you by mail. Now I know this is a lot more difficult then it sounds because this is an international forum and you would need to consider shipping charges, logistics and packing and also not every manufacturer would be willing to cooperate in this idea.
But imagine you can (for example) get a Basic Alox Victorinox in a unique colour that cannot be bought anywhere, it could even be just a one-time welcome gift instead of a yearly recurring subscription. Then I would easily be willing to sponsor the forum with 75 USD while the costs of the product and shipping will be not more then 20-30 USD. Meaning the forum gets a net. donation of 35-45 USD a year per subscription. I am not sure how big the market is for this kind of subscriptions but I imagine there would be quite some collectors wanting to add a an unique piece to their collection and support the forum in one go.
Again, not the most easy solution but maybe something to think about.