I use Fusion 360 for design and CAM for my MPCNC. The most popular (and probably user friendly while still having a decent number of features) solution is EstlCAM. My guess is that over 90% of the MPCNC users on this forum use one of these two tools. There are a few other nice design tools for CNC work, but they tend to be $$$ and the CAM solutions might be more of a struggle.
Similar to 3D printing, there are three aspects to the job, 1) design, 2) generate g-code and 3) deliver the g-code. Generating the g-code for CNC use is referred to as CAM and is analogous to what Cura Slicer does for 3-D printing. CAM is often split into two parts, general CAM and a postprocessor that generates specific g-code for a specific firmware (Marlin in your instance). The graphic in the first post in this topic will give you a map of your choices, and the Software Workflow page may also be helpful.
In my opinion, Fusion 360 is a more feature rich and flexible solution, but has a significantly steeper learning curve. If you use Fusion 360 as a solution, the FlyFisher postprocessor is needed and can be found on links in this topic.
As for delivering the g-code, RepetierHost and SD cards cover a majority of the forum users, though wireless solutions like V1pi are also popular.