I'm luckily enough to work on a small team like the one you described, and yeah - our trello board isn't fully fleshed out. We can put vague descriptions of what needs to be done and the team gets it done.
I think SMART goals are one of those rare times where an HR course writer unintentionally hit on something that some people need to hear. There's a junior engineer on my team whose goal was just, "I want to get better at infosec" - not measurable, time boxed, etc. by trying to at least hit one or two of the guidelines, they were able to flesh out this goal into things like "I want to attend a major security conference this year" and "I will study for, and achieve my Security+ cert".
It worked for them - and helped them clarify their broad nebulous goal into smaller specific and achievable goals - but obviously like all business/he things SMART goals aren't for everyone.