Yes, sounds reasonable. Although 30A is way more than a Pi and Arduino would need; choose something better matched to the load, or it won’t regulate well. Or make sure you are drawing 25A with something else at the same time.
Pi can be a bit picky about noise on 5V input. Check the 5V power with oscilloscope or digital multimeter on AC mode, to look for noise, and add a bit of capacitance if there is too much. In my tests, an rpi3 shows flashing red LED when power not good and CPU loaded.
Predict what happens if the UBECs or the other converter fail in a way that draws lots of current. You’ll want to stop that, or the cables could burn. Parameters that may be interesting in that prediction;
the maximum fault current of the power supply; while the supply is specified for continuous 20A at 12V, the fault current could easily be double that for as long as it takes to start a cable fire,
the maximum current of the cable between the power supply and distribution block, the distribution block terminals, and between the distribution block and the converters,
the maximum normal input current of the converters.
If you stay with the 30A converter, you might add an input fuse to make it safer; 30A at 5V is 150W (167W at 90% efficiency), 20A at 12V is 240W (216W at 90% efficiency), so it should be okay. But if you are pulling anything else from the 12V supply, do the math.
Hope that helps!