I take the utilitarian longtermist position to be that we ought to prioritize maximizing the probability that intelligent life is able to take advantage of the cosmic endowment.
I phrase it that way in order to be species agnostic. Given our position of ignorance about intelligent life in the universe, and our significant existential risks we face over the next couple centuries, it seems to me that we can right now increase the chance of intelligent life taking advantage of the cosmic endowment by increasing the chance that life exists beyond earth.
We can do this through directed panspermia, and calculate that with enough seeds emitted, evolution will be able to eventually produce more intelligent life with some probability that counteracts the probability that we destroy ourselves.
I think the decision is a difficult one, much more difficult than it’s been given credit, with the default being to protect the sterility or potential biome of planets during our exploration efforts. However, if our long term plan is to become interplanetary, then we already plan on directed panspermia. Why not buy down the objective risk that there is a universe void of intelligent life through panspermia now? Call it a biotic hedge.
I'm not a utilitarian, but if I were, I would emphasize quality over quantity. There are two ways in which quantity can harm quality. The first is when there's a trade-off and spending resources on quantity causes you to spend fewer resources on quality. So if you spend money and attention on implementing panspermia, you can't spend the same money and attention on improving the quality of life of sentient systems. The second is even worse: On those margins where quality is negative, quantity actively hurts the total. So you had better be really sure that the quality is positive before you spend resources on quantity. In the context of panspermia, I'd worry about the suffering and preference-frustration that will be caused by the project.
I'm not a utilitarian, however, so I wouldn't donate to such a project either way.