Marlin has M280 and M281 g-codes for controlling servos.
You will have to enable servos and recompile the firmware. I’m guessing, since your plotter/cutter will not be set up the same way as your Lowrider was, you will need to recompile the firmware for more reasons than just servos. I don’t know all the in/outs of Marlin servos, but I’m guessing that you first have to enable servo support by uncommenting the following line in configuration.h and setting the number of servos to 1:
//#define NUM_SERVOS 3
Then you will need to add the following line to pins_MINIRAMBO.h:
#define SERVO0_PIN [some number]
The “[some number]” is the pin number of an available PWM pin. Note there are not many available PWM pins on the Mini Rambo. The best choice is also used by the display, so if you want to use a display, then you will have to jump through some additional hoops. If you are not going to use a display, you are golden.
If you have a cheap 9G servo, you can set up servos and test sending g-code movement with what you have now. No need to wait until you have your machine further along.
As for GRBL, things get a little more complicated. There are a number of implementations of GRBL, each targeting a set of control boards. It is unlikely the versions you reference above will run on a Mini Rambo. It is likely that these versions will only run on the CNC shield.
If you are going to be using Fusion360, then you could set things up as Laser M106/M107 commands. You would then have to search and replace all the M106 commands with the Servo down command., and the M107 commands with the Servo up commands. It is a bit hacky. It is also possible for someone to edit the Marlin postprocessor to automatically insert the Servo commands in place of the laser commands. If you go down the Marlin road, let me know, and I’ll see if I can figure out how to modify the postprocessor to make the conversion.