would require GADS of time
Well, not really. What is required, in some cases, in GADS of attempts. How many organisms are alive at any one time on the planet? If we stick to multicellular it probably (I didn't count 'em) amounts to trillions. If we include microbes the number is BIG. Each of those reproduces and each time there is another roll of the dice. Now to get enough attempts to produce results it may still require rather a lot of time but when generations are hours, days, weeks and maybe a year long a million years is a long time.
As for the cambrian, there are a lot of different body plans that appear
in the fossil record over are geologicaly short time. However that short time represents (at a guess) a few 100 million generations (we don't know the life span of those critters) each with GADS of creatures in them.
In addition, there were things liveing
and evolving before the Cambrian explosion. The very skimpy fossil record doesn't help trace the lineages back much though.
Now look at the creatures that are preserved in the Burgess shale. While experts have managed to tease out the details and the differences they are not exactly a collection of gorillas, gyre falcons, groupers and the like are they? The diferentiation has just started.
You are reading too much into the popular picture of the Cambrian without knowing enough about the details.
Before you make decisions on your personal incredulity and lack of knowledge maybe you should look more closely at the information that is available. If you take a 5 or 10 million year time period where there is more of a fossil record you see very significant changes. The fact is that pretty big changes can happen in a short time.