@GrudgeNL here are some more solutions that worked for some people, you could put them in your OP.

Install RS 2.5 in a separate new folder; Copy the RS files into the RTW folder steamapps/common/Rome Total War Gold and overwrite where needed. (You can backup your RTW folder by making a copy of it if you want before) Open Steam and go to the launch options for RTW. The Desert Kingdoms Culture Pack introduces four new playable factions to Total War: ROME II. The Kingdoms of Kush, Saba and Nabatea fall under the African/Arabian subculture, while the Masaesyli are of Numidian origin.

Language Localization

If you want the game in a different language than English, wait until it's fully downloaded before you change the language to your preference. This solved the launch crash issues for some people.

Desert Dispatch

Visual C++ Redistributables

FIXED - I have fixed my issue. When I launch the game from steam a steam window will open saying that its launching, then it disappears and nothing happens.
I opened up my local files - C:Program Files (x86)SteamsteamappscommonTotal War WARHAMMER and then opened _CommonRedist and then opened vcredist. I then open 2010 and installed both the exe files vcredist_x64.exe and vcredist_x86.exe. I did the exact same with the 2013 folder; installed both exe files.
Now my game works.
I also found the 2015 ones and installed these.
https://www.microsoft.com/en-gb/download/details.aspx?id=48145

Windows Service Packs - make sure your Windows is updated (PC user's responsibility)

Go to Start -> Control Panel -> Windows Update

Steam Rome 2 Desert Patch Download Loop 1

Check manually if you're missing any Service Packs or other updates.
I could provide a link for you, but it needs to be in your Windows' language.. so you will need to google for it yourself.

Steam Rome 2 Desert Patch Download Loop 2

D3DCOMPILER fix (only recommended for advanced users)

Total War Rome 2 Desert

Ooooooh fixed it! See the following link and read to the bottom. Basically you have to delete the D3DCOMPILER _43.dll in both System32 ansd SysWOW64 folders (this means nothing to me) and then reinstall Direct X. I won't try to explain it, just read this forum....
https://forums.marvelheroes.com/discussion/6661/d3dcompiler-43-dll-is-either-not-designed-to-run-on-windows-or-it-contains-an-error
Enjoy!