Buck converter after 50Hz transformer
Your design is good. It is OK to use 50 Hz isolating transformer and then buck DC-DC stage. However: you should take care of interference and isolation. This may be not so easy, if you do not have good enough PCB technology at hand.

You can not make your buck stage on one IC because of high voltage / power requirements. You can use ready-made buck controller (for example, LTC3810) and two small power MOSFETs rated for 80 V operation. LTC3810 can regulate its output in range from 0.8 V to 50 V. You have to select the proper feedback correction to run the switching regulator in such a huge output voltage range.

Additional benefits of this design are: built-in over-current protection and soft start.

The problem is: you need a good PCB technology to make this design work fine.

As for the switching noise - it is only a matter of proper filtering. I have designed a lot of very sensitive analog and RF circuits, all containing switching regulators. It is enough to filter the output by one or two stage LC filter with MLC capacitors and low quality factor inductors. I suggest to add one common mode choke on output of your DC-DC stage and another one - on its input.

The linear regulator after the switching one does not help against a switching noise, it is a myth. You can check specs for linear regulators for AC suppression: it is pretty low above 0.5 MHz. And we are talking about noise / interference from the switching frequency up to 100 MHz, at least. So cheap passive components (inductors, ferrite beads and MLCC) solve this problem.

The only drawback of using 50 Hz transformer compared to fly-back switching design is: the capacitance between Mains and your DC output is ~100 times larger with 50 Hz transformer. It may cause problems in RF applications.

Once again: your major problem in this design is the PCB technology (2 layers is absolute minimum, 4 is good) and proper PCB layout design.