Since I have been thinking of lasers that has brought up the limitations of my current controller.  The advantage of the current controller is the ridiculously low cost, but it is very tight on I/O pins and generally not a very solid setup with most connections being bare pins without any sort of a key or lock.

One idea for an upgrade is the Cronos Maker V3 board. This is a 32 bit controller complete with 4 stepper drivers, support for loads of I/O, can directly drive the spindle, and has an optional offline controller that can be used to drive the machine around for $40 to $56 depending on the configuration.  The big advantage here is the included housing (nice) and integrated spindle driver that should allow you to not only control the spindle from GRBL, but if I understand it right will allow you to use a single power supply to drive both the spindle and stepper motors.

It would be nice to have another stepper motor axis, having only 4 does not allow us to add a rotary axis to the machine.  With a rotary axis we can etch the paint off of a hydro flask, or we could build a small chuck that would allow us to turn the machine into a mini lathe.

The BIGTREETECH SKR v1.4 with touch screen display looks like a great upgrade at $68.  This improves on the Cronos by adding in a SD card reader and touch screen, but while it does have a heat bed driver that is probably not strong enough to drive the spindle.  On the upside this can become a fully stand alone controller but you will need to use something other than GRBL as the firmware.  Marlin would probably work just fine.

If that all sounds too pricey then the KOOKYE SGen_L 32-bit Controller at $20 is another option that is much closer in price to the original GRLB controller we have been using.  The downside here is you get nothing with it at this price, not even motor stepper drivers.  And without some sort of LCD display you can't use it as a stand alone controller since there is no way to select the gcode file or to set zero without a computer.  Once you put all the missing parts back in you may be right back up to the price of the other two controllers.  However if you just want to upgrade your existing controller this may be a viable upgrade path.

Keep in mind I have done about 15 minutes of research on each of these controllers.  You should take your time and investigate what firmware works with each and read reviews before settling on one.  And of course the current controller gets the job done for a very low cost.

Here is a new one, the Makerbase MKS Robin Nano V3.  It has usb thumb drive support, so no need to use sd cards when moving designs around.  It is full 32 bit, comes with a large touch screen with a thumb wheel, has support for 5 axis, and has support for switchless homing, wich may or may not work well with a cnc machine.  The price varies, but for $47 you get a wifi module as well as 5 TMC2209 stepper drivers and the MKS TS35 touch screen.

With all of these controllers you would want to verify what firmware they run and how you would go about customizing it.  They will be setup for 3D printers for the most part so you want to have the power to change things as needed for your own use.  I suspect the BIGTREETECH has the best firmware support, but it is possible the rest work just as well with Marlin 2.0