I'm not convinced that a fine is going to change someone's behavior. It may put them in a regretful financial bind, but it's really a short-term pain. Spending a week cleaning trash is definitely something they will remember! (and for sure they will complain to friends and family who will then maybe be less likely to litter themselves)

Singapore has plenty of people littering. They also have a massive army that is constantly cleaning.

Yes, if caught (unlikely) you will be severely punished. But that’s not the reason Singapore is so clean.

So when the person you tell to do community service says, "No", then what?

SF has no willingness to actually punish crime. The cops won't show up to stop shoplifting, bike theft, or any number of other crimes, and now you think there's going to be a trash patrol that is going to actually do something?