There is of course lumber, which can sequester CO2 for decades or hundreds of years. Lumber sequestration currently only amounts to ~1% of global annual carbon emissions, but regionally it can be big (9% in Sweden) [1]. Lumber might have a bigger impact if purposefully employed for sequestration and prioritized over other carbon positive building materials.
Does the evidence support that people's goodwill get "used up"?
It seems as plausible a theory to me that, when people get involved and take direct action, it ends up driving them to take further actions. Being personally invested in the result may drive more action rather than using up the goodwill.
(To be clear, I agree that we should always encourage action with the highest impact, and if tree planting actually does nothing to help solve the problem, then we should direct people to other activities. I'm not really jumping in to defend tree-planting initiatives, but more to question the assumption that people have a fixed amount of effort that they're willing to put into a given problem)
That's a good question. I assume it will be both for different groups of people. From my personal experience it seems to be that way, but I'm unaware of any studies on the subject.
Then there's the problem that there's no significant process that turns trees into sequestered carbon for longer than the trees' lifespan.
The reason it's important to keep saying this is that lots of people participate in these tree planting drives. They feel all accomplished and that they did their part. But if that doesn't have any real impact on the problem, then we've done lots of work, used up people's goodwill, but not dented the problem.