If you sit down and study which types of plants sequester the most carbon, and plant a forest comprised entirely of that species, you run into a few problems:
First, parasites tend to be much worse. Whichever parasite prefers that particular type of plant will have an enormous food supply and a subsequent population explosion, with dire consequences for the entire forest. As the plants die, they will release the carbon they've stored.
Second, different plants prioritize pulling different resources out of the ground. There isn't a simple recipe of ax carbon dioxide, bx water, cx nitrogen, dx phosphorus etc = x kg carbon sequestered. Some species need more nitrogen, some need more phosphorus, some need more water etc. Different areas of the soil will have different concentrations of each resource the plants need. By having a diverse population, you will better utilize the soil's diverse resources.
https://phys.org/news/2018-10-species-rich-forests-carbon-mo... DOI: 10.1126/science.aat6405
In the study, the monocultures they studied included monocultures of bamboo and eucalyptus, which are extremely productive monocultures. These plots were still less productive than diverse forests.
You're correct about algae scrubbing carbon faster than forests, however algae to do not sequester appreciable amounts of carbon. It's released back into the environment nearly as quickly as it is removed. Trees, on the other hand, sequester lots of carbon in their trunks and root systems, so they are an important consideration with regards to anthropogenic climate change.
Re: soil diversity, by all means pick plants/trees that are suited to that soil, as long as they are especially effective at trapping carbon.
Fair point about algae.
Its irrational to continue to neglect natural systems in a world where their capacities to sustain diversity have been overloaded by anthropological targets.
There are certainly ecological arguments for natural forests v. artificial carbon-sequestration-optimized forests, but if maximum carbon sequestration is the goal, then the rational answer is to focus on those plants which sequester relatively large amounts of carbon instead of waiting for maybe a natural forest of not-necessarily-optimal plants to maybe grow on its own.
Also, last I checked, algae is still Earth's primary oxygen producer (and therefore implicitly the primary atmospheric carbon scrubber).