Driver is way more important than setups in GTS, so we already have what you are looking for.
Many people make the mistake of watching world tour/top split streams and presuming racing will work the same way at their level - that their car won't be able to catch the car that's dominating top split based on its BoP.
That's not true at all - until you are in the tippy toppiest of splits, you are not against drivers that can keep a car at the grip limit for an entire race, quite far from that in fact. You'll notice the people driving the meta cars you watched in top split are 2, 3, 5 seconds slower than the alien - yet the aliens driving the non-meta cars are within a second and a half.
What does this mean? It means you can be quicker than whatever else. In fact I think there's a tangible advantage to driving lesser known cars. I've used the gr.3 Genesis quite a lot - it is not a car that excels at anything, and 2nd gear is dangerously drifty, however people aren't aware of the characteristics of the car and it can surprise people with its balanced performance.
I drive the NSX in both groups for the same reason - no one is prepared for how good of an all rounder the gr.4 is, it will outhandle the FFs and it will hang with any FR or MR just about anywhere. Easy car to drive, so easy car to race. The gr. 3 is more of a specialist deal, but people are more familiar with the R8, to which it is similar, even so the NSX will hang with any gr. 3 car and honestly leave a lot in the dust if managed properly.
So, IMO, you're asking for tuning to do something you can achieve alone with skill, and properly learning to drive your car of choice. I hate the gr.3 GTR - I always slide it, and I can't go fast in it because I haven't put in the time to understand it. Hopping over to the top16 meta car just doesn't actually make you instantly faster.
I'm mechanically inclined, a real car-head and student of engineering, and I hate tuning. It's tedious and laborious and I just wanna hit the track. I'm sure that's not an unpopular opinion. Just be happy they let you tune at all, and find a private league that allows it. All I use it for is putting 5 degrees of camber on the front of a road car, stiffening it within an inch of its life, locking the diff and sliding it around.
And drive whatever race car you want. Meta might dictate powertrain to a degree sometimes, but you always have a choice. Screw what the aliens are doing to save every .005 and just drive what you enjoy, I guarantee if you put time into and master that car, you can pass whatever else you come across.