You've pretty much hit the nail on the head. In the images that I've seen of the 3018 bundled software (and that is the limit of my exposure to this) - it shows that the manual controllers allow you to load ".nc" files from SD-Card, or issue the same file from the grblControl software - just like you would with the early reprap 3D printers (no surprise as the GRBL controllers all derive from the same architecture). That gives you the flexibility of using your own design software and whatever CAM process you desire - the only stipulation being that the output is compatible with the GRBL subset. That's all do-able, and can be pretty cheap (i.e. free). But much depends on what design software you choose to you - and that depends on your use-case.

Re. the "quality of the low level software they give you with the machine" - there's two aspects - the firmware installed on the microcontroller (Arduino?) - which will likely be derived from the GRBL project firmware - it's basic, but it works for what it is - millions of 3D printers use this; don't expect the world and you won't go far wrong. The biggest challenge will be the performance of the GRBL interpreter but you're asking probably less from a 3018 than from a 3D printer in terms of throughput, so probably not much to got wrong. The PC-side software... yeah, a little basic from the screenshots I'm looking at, but an interesting (and somewhat trivialised) entry into the CNC world.

You could... and I say this just to cover one base, replace the electronics much later down the line and use the hardware to support a more "conventional" (I'm not sure I'm using the appropriate word there) approach to the more robust world of CNC and controller software, but in honesty you'd probably outgrow the 3018 hardware a long time before you'd want to go that far.