You need either an H-bridge, or a DPDT relay plus a flyback diode to protect the drive mosfet. If you use a relay then you should be careful to reduce the motor current to zero and pause a short time before reversing the motor, otherwise you will get arcing over the relay contacts.
Duet WiFi boards have unique MAC addresses. Duet Ethernet boards used to use the same default MAC address (along with Duet 0.6 and 0.8.5 boards), but in 1.19 series firmware the Duet Ethernet MAC address is derived from the unique MCU ID. So two or more Duet Ethernet boards will almost certainly have different MAC addresses when using 1.19 series firmware.
This is essentially what the "lookahead/planner" in the firmware already do. The jerk or instantaneous velocity change is the setting most related to this. The planner looks at the velocity change vector (dv) and determines how much it needs to slow the head down in order to be within this parameter.
This is a big problem for 8 bit controllers (obviously the Duet is 32 bit) as their move buffer often runs out on fast machines, especially when doing circles. In those cases, since the planner has no moves to compare the current velocity to, it comes to a complete stop at the next point and then picks up again from there.