Welcome to the forum!
I think September is a bit late for the flowers. Your ideal flower month is August. September will still show some flowers but mostly they start blooming last two weeks of July already so the peek is most definitely early to mid August.
You cannot enter the Richtersveld Tranfrontier Park (RTP) via De Koei. That is an exit only route. (By the way De Koei is not existing anymore.) From the gate where you sign in, you have to proceed to Sendelingdrift to register/pay etc. From Sendelingsdrift you must approach De Hoop via Akkedis Pass. The difficulty of the route varies from season to season. Most of the traveling in the Richtersveld is via ravines, so the routes are exposed to lots of erosion. By September most of the holes/dongas will be filled up or bypassed so I can confidently say your Disco will breeze through. Still you do get the odd spring downpour so check as close as possible before your trip. Grading is a 2 or at the most 3.
The Eksteenfontein-Helskloof Pass to Vioolsdrif route is a bit more challenging than the RTP itself but still not anything to worry a Disco3. On the reverse route - from Vioolsdrift back to Eksteenfontein there is one sharp accent that will require low-range, but first mentioned way round it's an impressive down-hill not to worry about.
Myself will be returning from Kgalagadi/Ai-Ais/Richtersveld end of August so you can contact me then. I intend to also use the Gamkab 4x4 route. I have not done this route but have been in contact with people that has done it. Again it is much the same as in the RTP. Mostly a 2-3. In one of the latest off-road magazines, somebody claimed it to be a 3-4, but they were totally inexperienced (female) drivers, towing a trailer so I have my doubts about the grading. It is not any driver that will successfully do a 4-grading towing a trailer. You are most welcome to PM me for more info. For proper 4x4 information to the RTP area, try this
Enjoy the preparation and all the planning - for the RTP it is necessary to plan well.