Rally games and F1 are too much specialised to be an alternative I think.
I don't see the need to dismiss certain racing titles just because they only focus on one discipline. Good rally games are a ton of fun, and F1 games usually have very good attention to detail and often feature a number of circuits you don't find in other racing games. I would rather have 5 very different racing games than buy every version of one franchise.
Driveclub feels too much like an arcade racer with limited cars.
Driveclub's physics feel more realistic to me than GT6's. Sure you can change set up parameters in GT6, but it doesn't produce anything like real world results, so it's pointless. Put DC on hardcore mode, so it takes away the grip and brake assists, and it feels close to Forza 5, and significantly more believable than GT's wooden physics. As for cars, GTS will have 137 cars, a large amount of those will be VGTs and duplicates (there is literally 4 versions of the same lancer...). By comparison, Driveclub has 114 cars, with no imaginary cars, and no duplicates. All unique, and many very desirable models not present in many, if any, other games. DC has a far superior car list to what we know of GTS.
I'm only just a little bit more serious than a casual racer I guess but the more realistic weekend setup sounds great but for me in theory but isn't really convenient. With the limited time I sometimes have I like to just fire up GT6 or Forza 6 and do 1 to 3 races that take 10 minutes each and switch it off again. I felt like I couldn't really do that with Cars as I needed to first drive few laps, make changes to the set up, test again, qualify, race... Since then I haven't touched the game.
In this paragraph, you sound like the exact person Driveclub was made for, but I'm glad you find Forza suits your needs. I agree that Pcars is a much more serious sim, which requires learning how to properly set up a car. Not really well suited to casuals.
Gran Turismo stands for quality.
Those days are long gone my friend. The last GT game that felt like a quality purchase was GT4. GT5 and 6 were plagued with bugs and the physics were laughably backwards from reality. The inclusion of standard cars and tracks that had not been so much as touched up since the PS2 days, in a 2013 release, was the exact opposite of quality.
I thought to have read it would also still have a career mode. But I assume it won't have as much depth as I would like.
There's no career mode. There's a handful of tutorials, like licence tests, and the normal arcade mode. Other than that, it's online only. As I said previously, it's the Battlefront of racing games.
The fact that it is the first GT game for PS4 will be enough to reach good sales numbers I think.
Depends on what you (or PD and Sony for that matter) consider to be good sales. Compared to Pcars I bet it'll sell well, but compared to every other GT game I bet it doesn't at all. Also, the game's success will depend entirely on how many people continue to play online, as the entire game depends on that. I can't see the majority of people who buy GTS being long-term online racers. Only a tiny niche of racing game players are interested in racing in leagues, most want a story or career mode. GTS doesn't offer that, and so I bet most buyers leave it on the shelf or return it shortly after buying.
As for the rest of your post, I'm glad Forza 6 fits your needs, it is, by any measure, an excellent game. I too hope T10 stays true to the recipe, because now that PD is trying way too hard to make GT different, Forza is the only series to turn to for that specific type of racing experience.