Yes he might make episodes longer, but there is a certain cut off with that if he wants to maintain his preconceived structure. The difference between the upgrade to 3090s and this is that all of the key system components have had an upgrade, better ram, better CPUs, better MoBos, better GPUs, it would probably cost him $10-15,000 to fully upgrade and build 3 top end rigs, two for rendering one for work.
Thing is he could always keep his current rigs running in addition to the new ones. That way he could have a couple dedicated to long form animations chugging away in the background while the others could focus on the other stuff. Given that he won’t hire any help, it’s the only way to use the money he earns to increase productivity.