Great overview. Worth adding some population genetics: Multicellular organisms typically have small effective population sizes and reproduce slowly in comparison to bacteria. Selection has a hard time “getting a grip” on variants with very weak effects on fitness. Drift becomes much more important.

Bacteria have high population sizes. Selection can be quick and brutal. Low levels of “code of unknown function” in bacteria is perhaps related to replicative efficiency. Fast DNA replication is highly advantageous in nutrient-rich environments. No space (or time) for junk DNA.