How should a contributor gauge whether to make big changes to “do it right” or to do it a little hacky just to get the job done?

When you work in enough diverse codebases, with enough diverse contributors, you begin to understand there isn’t one objectively right way. There are many objectively wrong ways to do something. Picking a way to do a certain task is about picking from tradeoffs. A disturbingly common tradeoff is picking rapid development over long term maintainability, but that isn’t not the right way to do it in a competitive space.

Needs change over time and certain tradoffs may no longer apply. You’re likely to see better success making lots of little hacky fixes until it’s not a hack anymore because you’ve morphed it slowly over time.

Version control, git et al, allows you to make multiple commits in a single PR, so you could break the changes up to be more reviewable.

Hear, hear! I would add that it multiplies again, again when other people are actually using the product. Engineers famously build tools for engineers which can leave something to be desired for the layman.