Just donated a small amount. I think this is a nice service, and can think of a couple of friends who struggle with programming (professionally!) but are too embarrassed to ask and progress, and could make use of this service.
I hear the pain-points about the donation-driven model not rising to the task, but I do wonder how prominent that particular call to action is on the site. On one hand you don't want to bombard your users with annoying jimmy-wales-style boo-hoo donation banners. On the other hand you don't want your users to be completely oblivious to the fact that their favourite service is about to be shut down if everybody thinks that "it's ok, they probably have enough money / I wouldn't make a difference anyway".
Have you considered having a discreet yet prominent progress bar on the website, showing your monthly donation needs/goals? This could act as a good prompt for users to consider donating. One suggestion would be to accompany this by an information button, which when clicked takes you to a page explaining the different costs that need to be covered, where your money goes when you donate, and a detailed breakdown of what is currently still required / missing. Optics-wise, I would also try to find a way to differentiate between "large" donors and donation "trickles". E.g. you could color code on that progress bar different "tiers" of donation (or perhaps display this separately as a pie-chart in the breakdown information page). The point of this is to show to people who might be tempted to donate "small" amounts but are sitting on the fence because they don't think it'll make a difference, that it "can" actually make a cumulative difference, and push that kind of user past that threshold and donate "something".
Alternatively / additionally (you may do this already), if and when you see that a user has already extracted value from your service, such as, e.g., when they have submitted their 100th exercise, you could show a "congratulations" banner that says that you hope they're finding the service useful, and politely asks them if they could consider donating (oftentimes worded as "buy us a beer or two"). If they decide to donate, then after their donation, you could also have a tickbox at the bottom of this banner to ask in a respectful manner (i.e. not a dark-pattern-worded / confirmshaming one) if they would like to be reminded again after some time / number of exercises, or if they simply prefer to make a one-off donation and that's fine.
Finally, depending on your resources, if people "do" buy you a "beer", it would also be a nice touch to thank them for it, in an as personalised a manner as possible. Better yet, you could give them some sort of 'reward' / 'recognition' on the site; e.g. I think something silly such as revealing a hidden feature that allows a user to then tick a box to add a silly 'holding a beer' overlay to their avatar icon (in a similar way to how stackoverflow gives 'hats' close to christmas time), would actually go a long way to motivate people, after seeing these beer mugs on other people's avatars. And it's "silly" enough that people might donate to get this just for the fun and silliness of it.
The donation model is an odd one. Obviously, not everyone donates, but I think given some prompting and lack of friction in doing so, there's a significant number of people who would be inclined to donate out of a sense of gratitude, but who don't do so if they haven't been given a reasonable opportunity / visibility to do so, or some sort of confirmation that their donation doesn't just go to greedy shareholders on top of some sort of pre-existing dodgy revenue stream, but makes an actual difference to real people running the service.