That is a fantastic idea but as mentioned, you’ll want a version of Marlin which also has spindle support enabled. I don’t know if that’s and “AND” thing in the Marlin setup or and “OR” thing. If it can be enabled along with keeping all your 3D printer setup enabled(Hot ends, temperature sensing, etc) then you don’t have to worry about delays, just set the minimum Laser power(Spindle RPM) to that which just turns the laser on but not burning. This way when burning it won’t turn off on moves and won’t require fan spin-up deltays.
As for G21, that is what tells the firmware what measurement units the gcode is in(metric or imperial) so it surprises me that is invalid.
It would be a very cool hack to remove the Ender 3 hotend from the carriage, add a tool change plate and easily switch from laser cutting/etching to 3D printing. You get Z axis focus adjustment with it. I’m VERY interested in how this goes for you as I have a couple of lasers I can use and the Ender 3…
I’ve read that updating Marlin requires access to the controller board, uploading the Arduino bootloader and from there all updates can all be done through USB. This hack would be worth the effort.