Purely from a technical perspective, the problem set of any potential algorithm, even an incredibly naive one, would not take an appreciable amount of time to complete.

The only problem you'd run into is if you somehow ended up with a solution with factorial complexity, but that's incredibly unlikely.

for each plot:
    while there is no seed planted:
        if there are any adjacent seeds, make sure you iterate over appropriately tiered seeds first
            if the plot has proper dispenser coverage:
                plant the highest tier, highest level seed you've got

And that's really it. To make things even more simple they could require you to have the seeds in your inventory rather than in your seed stash.