Fitting a silicone sock to my heatblock stopped similar small variations I noticed, it's probably worth trying one (or kapton tape layers) to rule out stray drafts etc that can make temps wobble.
And remember that PID is a feedback system; so it will wobble slightly in response to changing demands (say moving from a big area of constant rate wall/top printing to suddenly printing a lot of support with more travel than print moves), or moving from the cold side of the heatbed to the side out of the drafts, etc.
Each time the load on the heatblock changes you will see a small fluctuation before the controller brings it back to stable, and tuning PID's is a compromise between getting the fastest possible response, but without overshooting and possible oscillating around the set temperature without stabilizing.
That display should be electrically compatible with the PanelDue controller if you order it with the resistive touch screen; however the pinout of the 40-pin connector is different, so it is complicated to wire up.
Are they stepstick-type drivers, or large external driver modules with opto isolated inputs? If the latter, then given that those are usually intended for 5V drive and they only get 3.3V if you drive them directly from the expansion connector, it's quite likely that when you drive 2 of them form a single set of pins, the drive voltage drops too much. In which case, do as Tony says. Your interpretation of what he meant is correct.
I suggest you use a multimeter to check the voltage between ENA+ and ENA- on your external drivers. It should be 0 when you start the Duet, changing to almost 3.3V when you first command the X motor to move.
Similarly, measure the voltage between DIR+ and DIR-. It should switch between 0V and 3.3V depending on which way you try to move the X motor. For example, if you send G91 first, then G1 S2 X10 will set it one way and G1 S2 X-10 will set it the other way.
If those are both working, then although most external drivers work OK with the 3.3V signals provided by the Duet, it's possible that your drivers need a full 5V.
Did you mean with the external stepper driver interface board? If so, yes, if the servo system also has step/dir/en inputs compatible with that board (5VDC signals). But the answer can be very complicated because servos can have specific configuration requirements.
If it's a standard hot end with a cartridge heater, you can just replace the 12V cartridge heater by a 24V one.
If not, then here are 3 options:
1. Power the whole printer from 12V instead of 24V.
2. Power the rest of the printer from 24V, but as a 12V power supply for the hot end heater and anything else that needs 12V.
3. Buy a 12V buck converter to generate a 12V supply from 24V.
For #2 and #3, connect your hot end heater between +12V and the E0- pin of the E0 heater terminal block.
What voltage does your bed heater need? If it is 12V only then you will need a 12V power supply to run it. Bed heaters need a lot of current, so using a buck converter to generate 12V for a bed heater doesn't make sense.
Just connect a 1N400x (where x = any digit 1 to 7) in parallel with the brake. Important: the cathode of the diode (the end with the stripe) must be connected to the positive connection of the brake solenoid (the end that goes to the fan connector pin marked V_FAN).
Here is a sample schematic. The mosfet in this diagram corresponds to the fan driver mosfet on the Duet, the "12V" label in that diagram corresponds to the V_FAN pin on the fan connector, and the "Drain" label corresponds to the FAN- pin on the fan connector.
I really wanted a rotary encoder to dial in the first layer but after using the Duet WiFi and converting my core x/y to the mini differential IR sensor and building a delta with a delta smart effector sensing accuracy is so precise my prints start up great. Occasionally I use baby stepping and noticed the setting stays in effect until I shut the printer off, at least it appears to work this way. Hard to tell since things just work so much better with the Duet WiFi.