Actually, it's mentioned several times in the docs that you shouldn't think that backlog build-up means you have to crunch―or more specifically, IMO, 'backlog' isn't a thing here at all. If you can only put in thirty minutes a day, then do it and then close the app, this doesn't change because of a missed day. The cards will be there the next day, it's not like they catch fire when you skip a session. Stressing over the backlog would likely only make your mind work worse.

Instead, change the limit of cards to review to the number that you can get through in the time you have.

(Unless your college schedule requires you to actually crunch some number of cards every day? I don't know if that's a thing, but using spaced repetition in such a case would seem ill-fitting anyway.)