Exactly, these are geological processes that take thousands of years. The thing you are worried about is not a real concern. You would be equally served worrying about a sentient teapot uprising or getting forever clone-tortured by an evil robot that's angry at you for not building it. Mass dieoffs of plankton and cyanobacteria are simply not concerns that worry anyone informed.

The last time, really the only time this sort of thing occurred extensively was the K-T extinction event, which itself took tens of thousands of years. This is the fastest, most complete extinction event in history in which a massive asteroid, climate change on par with what we're experiencing, innumerable massive volcanic eruptions that each make Krakatoa look like a mere science fair project, and incredible sea-level changes all teamed up to kill off 75% of all living species on earth. And this took roughly 60,000 years. During this whole period, CO2 levels were notably higher than they currently are, but so were O2 levels. As it turns out, the extra CO2 caused by ocean anoxia also lead to a huge increase in vascular plant biomass, and trees and forbs more than picked up the slack, resulting in oxygen levels about 30% higher than they are now. Even then, "drowning on land" did not happen.

There literally cannot, will not, ever be a day, a year, a decade in which all of these species suddenly die off. Extinction events are in fact a process that can only take many thousands of years. If the thing you're worried about began at the same rate today, the amount of time it would take to get to the point of "drowning on dry land" would be roughly as long as the time period between humans leaving Africa and today.

Your fear is simply not possible. The snow crab mass death event was a problem better described as a crisis of carrying capacity. A heat wave that started in 2018 resulted in a simply massive increase in the population of snow crabs in the Bering sea, with scientists discovering a population boom an order of magnitude larger than the anticipated population increase. This carried the crab population far above the carrying capacity of their habitat, causing an inevitable mass dieoff and a population reset to below the carrying capacity. This is something that happens all the time in nature and is impossible to predict, but ultimately is not that scary. The equivalent of this phenomenon for plankton is called a Harmful Algal Bloom. The exact thing you're worried about happens many hundreds of times a year at the largest scale physically possible, and we don't really worry about these mass boom and bust events on a grand scale because that simply isn't how plankton work.

Relax, take a break, disengage, and touch some algae.