It is much easier controlling switching noise in a non isolated converter then an isolated one, so the easiest way to achieve this may be to have a simple push-pull continuously running fixed output isolating converter, followed by the variable output voltage switching regulator for the power supply.
In this case I doubt using an inverter for galvanic isolation followed by a switching regulator will help. Using one of the transformer isolated topologies should work well enough and a single stage will be more efficient.
If I wanted low noise, then I would use an inverter followed by a linear regulator.
Theoretically, what you say is true. When it comes down to the actual design, it is just much harder to achieve a wide ranging transformer isolated variable output converter then a two stage solution. The USB-C voltage can be 5v to 20V. Whenever you have a wide ranging input output, a single stage transformer-regulator that doesn't product massive ringing is extremely challenging. I am talking about noise that is really hard to contain to the supply. Designing the transformer will be hard.
If you have a simple continuously running isolator that converts 5V to 15V or 20V to 60V, that is a simple, trouble free design. The transformer is very simple.
Follow it with a buck converter that can output 0-12V from a 5V USB-C source or 0-50V from a 20V source is also very straightforward with either an off the shelf inductor or something like a simple toroid core.
Go for the challenging design if you like.