If I'm not mistaken, all current Subaru's only have 4 latches across the back seat.
The 2020 Ascent actually has 5 latches in the 2nd row bench: 2 each on the outboard seats and a curious off-center one in the center seat.
The 3rd row only has 2 latches on the driver-side seat, and those aren't even aligned with that seat's seatbelt.
In the end, we have two carseats in the 2nd row bench -- forward-facing on driver's side, rear-facing bucket in center -- and two carseats in the 3rd row in the outboard positions. All of them are secured with seatbelts.
This allows the 2nd-row passenger-side "40%" seat to squeeze by the bucket seat so that the older kids can get in the 3rd row.
This arrangement still allows for a very small adult (emphasis on the "very") to sit in the 3rd-row center position and a full-sized adult to sit in the 2nd-row passenger side. For the cars we were looking at the Ascent was unique in this ability. They really thought that part through.
However, the 3rd-row passenger-side seatbelt is extremely hard to tighten against a Diono RXT. For some reason the buckle is slightly too tall and gets caught against the RXT before the strap it's connected to can be fully tensioned. I had to use that trick of tightening the seatbelt with the seatback inclined and then REALLY pushing the seat back up to compress the seat into the leather pads. And even then I'm really not 100% certain the seat won't pop forward under accident forces.
So that part Subaru did not think through properly.