What's your thoughts on replacing 5w-20 with 20w-50 on a Toyota Camry?

Very bad idea. The bearings will be starved for oil.

Oil viscosity is dictated by bearing clearance. Clearance of .001-.002 calls for 5W-30; clearance of .0005-.001 calls for 5W-20. There is a popular assumption that bearing clearances increase in an engine as the age increases, but I have measured clearances in a Honda engine with more than 250K miles on it and they were still within the original .001-.002 range. If you consider that the bearings are a metal sandwich with a surface of Babbitt metal as thin as the clearance, you will understand the bearing will be chewed up and spit out if the clearance increased to the .005 inch that uses 50 weight oil.

The white smoke on start-up is probably from deterioration of the valve stem seals, but it could also be oil being sucked in through the PCV valve. No matter what the cause, using oil that is too thick to flow through the close clearances of your late model Toyota engine will only cause damage to the bearings.

At this high a mileage you have nothing to lose. Because if you are burning oil, the car will not pass its net smog test without a lot of expensive work. So it is try that or fix the car.

As well, no car that has that sort of mileage be trouble free for 5 years. Th engine is wearing out already. If you are using this car for Uber or the like, you will be lucky to get a year from it.

Your neighbor is an idiot. 20W-50 will slow down oil burning. BUT it comes at a cost. It pumps slower at startup when you have the most engine wear. And it's thicker at warm up. So you get accelerated engine wear at all temps. Thicker oil doesn't cool as well, so parts run hotter. It creates more friction so it costs in gas mileage.

Even in San Diego, it flows slower at startup.

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The thicker oil will likely start to burn more rapidly as it oxidizes in the rings and prevents them from floating on the pistons. The VVTi system will get plugged up and you'll start getting cam codes. You'll have accelerated oil pump and oil pump drive wear as it was not designed to deal with that oil cold. You will have reduced lubrication on start up because it flows much more slowly. You'll have reduced gas mileage. You will have increased deposits and possible sludge build-up from the increased VI improvers in the heavier oil...etc.

Variable valve timing won't work right.