Calculate power supply for step down modules

You can use an appropriately rated bridge rectifier for converting the incoming AC to DC (you'll need a smoothing capacitor too also appropriately rated). Then you can use a DC-to-DC converter PROVIDING it's capable of taking a few hundred volts at its inputs. Many are capable of doing this but more often than not a DC converter might only be rated to use up to 48V DC or less.

If your AC supply is 240 volts then the peak DC voltage after rectification is going to be about sqrt(2) higher or around 340 volts and this doesn't consider over-generation or load drop-outs or lightning etc. Usually the smoothing cap is rated at 450V DC.

Also, the DC converter, if rated for an input appropriate for rectifed AC, will almost certainly provide isolation so it's relatively safe to touch the output terminals.

If you are a beginner though, I'd consider using a step-down transformer - less chance of burning your fingers.

Diodes are cheap using them will protect your switch. short-circuiting the ammeter will not damage anything

If you can live with a brief interruption when switching he relays aren't needed:

^{simulate this circuit – Schematic created using CircuitLab}

"A in" is ammeter sink "ground" on your circuit. "A out" is ammeter source "ammeter" on your cricuit. "volts" is "voltmeter" on your circuit.

This only works because the buck converter has input and output grounds connected internally.