Then you just get parties who don't promise anything.

Look at Doug Ford's 2018 election platform. Buck a beer and... nothing else. Look at the platforms for everyone who ran in your municipal election. I don't know where you live, but if it's anywhere like the cities I've lived in, the candidates don't really have a platform, but occasionally make vague statements like "I'm would like to address issues with housing" or "we should do something about the homelessness problem".

If every politician is given the choice between "vague statements that don't mean anything" or "legal consequences if you promise to do this thing that you actually want to do but circumstances change and you can't do it", they're going to go with "vague statements" every time.

Then they should not make (what would then be) a legally binding promise.

Yes, if this is what the electorate wants, they should present the contract and get the candidate to sign it before election night.

I think you'll find the electorate doesn't actually want that, though. The incumbent maybe has sufficient information to present an election promise, assuming they can implement it in the first few days before the state of the world has moved on, but the other candidates most certainly do not. Why would you want a politician making decisions before they have information? That would be downright stupid.

Instead of promising to “balance the budget”, have them produce a budget plan

Many parties do produce a vague budget going into the election, but it still doesn't account for unexpected events like the pandemic. And even though "balance the budget for 4 years" is a somewhat reasonable promise, creating a budget 4 years in advance is a terrible idea for so many reasons. Even just through the normal course of an uneventful term, things will change that the government has no control over, and if they can't react by modifying their budget that they made several years ago, then that will cause a lot of problems.

And if their promise is something vague like "balance the budget" and there are legal consequences to not balancing the budget, then the government would be encouraged to sell of infrastructure in order to make up any deficit they may have accrued. That's also bad.