Each of us is morally obligated to prevent avoidable harm. Regardless of how many options we're presented with or how happy we are with them, we have to choose whatever candidate(s) we believe will do the most good for or least bad to their constituency. This is part of our bare minimum contribution to society, and it should be mandated for the same reason that we're mandated to pay taxes.

So while I do personally kind of agree with the idea that you have a moral obligation to prevent avoidable harm inasmuch as you want other people to have that consideration for you, the sticky point here is a lot of the words you're throwing around are highly relative.

For me, I don't consider voting to be harm reduction. So...should I still be obligated to vote?

Mandatory voting strengthens democracy by making voter suppression impossible and strengthening democracy. The less people vote, the easier it is to take away their civil liberties. Making our voices heard is the most basic precautions we can take against fascism.

Except this doesn't make sense in the context of the real-world mechanisms for many election systems.

In the US, for instance, if you are a Democrat or a Republican in a state/district that is heavily populated by the other party, what you vote doesn't actually matter from a mathematical perspective. It doesn't matter if you vote because you will be drowned out by the other party. So why force someone to do something that's objectively futile?

Mandatory voting promotes engagement with politics. Sure, we can't stop people from just ticking boxes at random, but almost everyone is at least somewhat familiar with the popular candidates and truly random votes will likely cancel each other out anyways. The less time people spend thinking about whether or not they feel like driving up to the polling station, the more time they'll have to think about an issue that's important to them.

There are a thousand more ways to engage with politics than voting. I'm not sure why this is the way we should make compulsory.