Some items to remember in the design phase:

Account for the width of your Z module on the gantry and also the type and position of your end stops. On some sketching or on the actual CAD drawing rough in basic wiring pathways and?or cooling as it will bite you in the backside later for not doing it.

What thickness and type of material are you planning on using? What form of bonding are you going to use between elements and materials (welding, bolting, high tensiles epoxies)?

Remember to account for tool length of the longest tool you plan on using and include that into your Z module length. Also make sure that your Z module is wide enough to aid in the reduction of the cutting forces that will be feeding back up from the tool head.

A lot of this is simple but often over looked. I learned many of these lessons on this very sight when working on the design and functionality of my rebuild that changed into a complete design and build. Still can't thank JazzCNC enough for the knowledge and help he gave when i was starting up. Now that machine is enroute to the US and will be getting some upgrades and put right back to work.

Biggest suggestion is follow the KISS principle. As that will keep you from going down a rabbit hole that WILL give you grief in the build and in the ability to use the tool once built.