My 2 cents:
Some members (like me) would be happy to donate an extra amount from time to time. But I need to be prompted. I had no idea that the site was so heavily subsidised nor did I notice a “donate” button.
So I propose that on a yearly basis you circulate a message requesting a donation from those who are willing and able.
And the easier you make it to transfer money (PayPal? Credit card?) then the more likely folk will send some amount.