@smidlee
Quantifying the microbial biomass of the planet, and then multiplying it by a average generation for five billion years would be quite the achievement. I wouldn't even hazard estimations about all the permutations possible in that given period of time. I know you go right past hyperbole when you reach figures like that. Not only can't I do those numbers any real justice, but I can't even give an analogy that seems unreasonable. That said these numbers are about scale, and they might have nothing to do with the reality.
The more we learn about life the more we learn it isn't necessarily the complex riddle we continually make it out to be. To put it in simple terms you don't have to roll the dice a million times to get six fives in a row if nature has already fixed the dice to roll five more often then not. You could say where you see variables there aren't any. Just a natural process that has already predetermined the result. For instance can we say Helixes of DNA are unlikely when the atoms in crystals readily line up, or do we consider Benzyne impossible. Nature is full of novel complex structures outside of life. Just look at a common snowflake. Sure they are all unique, but underlying that there are basic common structures. Snowflakes come in classes, and each class is a product under which the snowflake formed. Water can form dozens of types of structures, but it is actually quite finite the difference is only in the nuance.
So when you look at life being too complex to be. The question is where does the inexorable conclusion end, and the nuance begin. From what we have seen of evolution it seems that Nature really cuts the cards in favor of life. Simple processes that can generate seemingly complex results. For example in your notion of evolution life may have simply stumbled upon the answer to get big by randomly one day hitting a huge perfect sequence. Well one prevailing theory is that it was more a product of environmental change specifically Oxygen in the environment reaching a level great enough for cells to produce Collagen. Which in turn allowed cells to bind together. Basically all life needed was a single simple gene to start making the compound. Once that happened life went to the races. Basically all life probably needed was for a prerequisite situation to arise, and once that happened life immediately responded.
Anyway life isn't probably the mathematical absurdity you imagine. Life is probably not only the logical answer, but it is probably the only logical outcome. Looking at Cell biology we see the aftermath of a war. Your cells aren't singular entities they are packed with organelles, and it is probably quite likely they were taken hostage in that war. So if life right out of the blocks can cook up so many versions its hard to see how the seeming complexity was so hard to come by.
