I left the scheme 4 months ago but I had a 2020 Peugeot Rifter LWB GT Line 1.5 blue hdi 130 EAT8 in Sunset Copper.
Before that I had a 2017 Seat Alhambra SE Lux 2.0Tdi 184DSG.
Seems like I’m the right guy to ask.
The alhambra had full 12 way electric heated leather seats and I would class them as reasonably comfortable, not the best but okay, while the rifter seats are manual adjustment I still found them about the same level of comfort even as you say the squab is a fraction shorter, where it does do a better job is in the corners as the backrest and bolsters do a very good job especially with the armrests compared to the alhambra.
I left the scheme early for different reasons and in general was very happy with the rifter, best car I’ve had for fitting loads of peeps, luggage, mobility gear and the kitchen sink in without missing a beat.
Hope some of this helps, any more questions just ask.