You can hack fix P0420 with oxygen sensor spacer; don't try anything else!

My thoughts are that modern cars (and planes and other equipment) push the limits on efficiency and require effective microprocessor/ computer control just to function.

Making some of the changes you are contemplating will upset the fine balance with unpredictable results. No one can really tell you exactly what will happen - you may be able to get by with some things, and not others. And if you do get by it will likely be for a short time. If that is all you want then go for it. But really the fact is that highly complex systems ( like priuses) are built with an intended operational useful life. Exceeding that is generally not cost effective. But many of us try.
150,000 miles and 10 years I think is an accurate assessment of Prius's useful life. Some will not make it, some will exceed it. Things like changing oil very frequently, or cleaning various components may help a little, but do not address the complex nature of useful life in dependable, reliable, meaningful way. Accepting that a vehicle is effectively obsolete after 150k/10 is hard but realistic.