I've had two RX-8s (2004 Sport Manual and 2005 GT Manual), a 2013 FR-S MT, and a 2018 BRZ MT with PP.
Let me start with the X-factor. The Toyobarus are not really the spiritual successor of the RX-8. I believe that is your hope, but unfortunately it's not true. The RX-8 feels more like a big Miata then the twins, which is a good thing. It's got a very stiff chassis with high torsional rigidity, allowing for softer springs. Combined with a double wishbone up front and it's hugely comfortable and compliant on the road, but with very good, fun, and predictable handling.
Second, the RX-8's rotary simply sings. There is nothing else like it. I haven't driven the 22 BRZ with the 2.4L yet (it's on order), but the 1st gen 2.0 sounds pretty rough.
Also, the RX-8's back seat is definitely easier to get into. I'm not sure that a carseat would fit better though because the hump between the two back seats is pretty big. And the suicide doors have pretty thick arm rests. I would definitely recommend test-fitting the car seat before buying.
All that said, the RX-8 is definitely more enjoyable to drive than both the FR-S and 1st gen BRZ. So why don't I still own one? A few reasons:
1. I like new cars. Any used example is going to have some miles on it. Eventually, it's going to need a rebuild, so you'll need to find a reliable rotary mechanic. I don't know any.
2. I like to tinker and modify my cars. Wasn't really worth modifying the engine on the RX-8. Nothing improved horsepower. You could go turbo, but would need a reliable rotary tuner. I don't know any.
3. The RX-8 gets pretty bad gas mileage and burns oil. I typically got 17-18 mpg and was always checking the oil level (annoying) and kept a quart of oil in the trunk.
Like I said, I haven't driven the 2nd gen BRZ yet. I understand it's more refined. Perhaps the chassis rigidity increase helped with that. But MacPherson struts are definitely very different in feel than double wishbone. However, if you've never driven a double wishbone car then you won't know what you're missing.
