Plus test is irrelevant for infant carriers because it is only done on seats which go up to 105cm or 125cm (approx 4 or 6 years). It doesn't mean an infant carrier is less safe.
Avionaut Pixel or Cosmo are lovely seats and vv good for smaller newborns. In general infant carriers are much better for newborns than multi-mode seats.
The 30 min rule is technically no longer a thing, it used to be 30 mins up to 4 weeks, 2 hours thereafter (for one stretch in the car seat without taking a break) but now they just say keep babies in car seats for as short as possible, it's necessary when you're in the car, but they shouldn't be used all the time as a seat or on the pram.
My advice - use common sense, be sensible with it. The shape and angle of a car seat is necessary for crash protection, but makes it difficult for babies to correct their position if their oxygen saturation begins to drop (which they can and will naturally do when lying flat). If you put your own chin on your chest, you'll see how it's hard to breathe like that.
Oxygen saturation is unlikely to drop especially quickly, unless your baby is very young or was premature, and even in these cases up to 30 mins in non-ideal position is OK. For older babies they tend to be OK for longer, and if you take them out of the seat even for just a minute or two it sort of "resets" the timer because their oxygen saturations will return to normal from them moving.
Don't use the car seat as a planned sleep location and don't use it as a seat within the house, but if they happen to fall asleep in the car, or you're going to pop in and out of a couple of shops, it makes sense IMO to use the seat on the buggy, or bring the seat inside and let them finish a nap in it, as long as they haven't already been in it for hours.