Tools that should be in your list and are almost imprescindible, Ade (airport design editor) with it you design airports (parking gates, taxi lines, runway etc. even you can place the .mdl objects with it) MCX (Model convertex, to check the things you créate. and many things more. a modelling tool, many options too. Seketchup, Blender, Gmax, 3ds max...
an OPT (object placement tool, there are payware and freeware). problably you want to create an objects library. the Library Creator XML is the tool. I you want to model your own objects an Image editor is of help to créate textures. Gimp, Photoshop and other are the tools. and it is almost the basic... Many tools available too for other scenery design fields:
Aibtc, for the creation of ai traffic for boats, ships
Ai Flight Planner to populate the sky with own traffic
Autogen creation tool to populate the city and litlle towns
Sode, for triggered animations, jeways creation with animations an many things more
And Many tools more indeed what make to develope for fsx very interesting
good luck.

At first it can be hard to get the process to export objects from modelling tool. create a library, compiling. and placing in Fsx, but one you get the process you will see it's not that diffcult at all, just time to follow the learning process.the rest is just a Little of hard work and good projects. and not get lost in the way.