Universal 3.3V & 5V power source for small device
It depends on the compromise you want.

If you want to maximize efficiency, you shouldn't chain DC-DC regulators. So you would use a SEPIC to get the 5V and a buck for the 3.3V, both supplied directly with the power input. But this requires 3 inductors and complex ICs. So it's expensive and takes a large PCB area.

If you want to favor price/PCB area instead of efficiency, do as you suggest, this is a good solution. You could even consider using a charge pump for the 3.3V -> 5V part instead of a boost, given the low currents required. It may save some more PCB estate and reduce potential EMI.

All in all, there is no bad solution, you just need to decide what you want to favor, find a few chips from manufacturers, estimate the component count/PCB area, do a few math to estimate overall efficiency, and choose.

There are a few tools from manufacturers that can help with this (I'm thinking about TI Webench and another one from Linear Technologies, but I forgot the name).