Surge protection in power supply
Given that [power] is coming from an external source over a long
cable (I expect the power/RS485 cable to possibly extend up to around
20meters or so), is it advisable to include additional voltage
regulation within the circuit?

Yes, there's a general rule of thumb: power every microcontroller through a &quot;local&quot; voltage regulator,
rather than try to &quot;share&quot; a voltage regulator between microcontrollers separated by long cables.
Once common approach is a &quot;Vehicle battery&quot; power and GND supplying power to numerous regulators.(a)
such as the John Deere CAN bus.(b)
The power rail on CAN bus cables (when they have a power rail) is almost always &quot;Vehicle battery&quot;, typically &quot;+12 VDC&quot; but is often &quot;+24 VDC&quot; (c)
(In such vehicle networks, the device needs to tolerate a load dump of much higher voltages and other weird battery-voltage transients).
Apparently a few Power over Ethernet (PoE) systems carry 5 VDC bus power, very similar to your proposed system; but PoE typically carries 24 VDC or 48 VDC bus power because:

Devices needing 5 volts cannot typically use PoE at 5 V on Ethernet
cable beyond short distances (about 15 feet (4.6 m)) as the voltage
drop of the cable becomes too significant,
so a 24 V or 48 V to 5 V DC-DC converter is required at the remote
end.

-- https://en.wikipedia.org/wiki/Power_over_Ethernet
Related:

Anything special to consider when running a 12v &quot;Bus&quot; throughout my house?
Voltage Drop and Safe Current Load on CAT5 Cable
https://en.wikipedia.org/wiki/42-volt_electrical_system
Power and RS-485 signal inside shielded CAT5 cable
1-wire verses CAN-bus for home sensing network?