Hello, sorry I didn't get the tag! Only just seen this, how annoying. I have been checking my notifications as well.
Remember your baby is going to be 3-4kg at birth and relatively quickly weigh around 8-9kg. In all honesty the weight of the car seat doesn't make a huge amount of difference. They get heavy and uncomfortable pretty quickly anyway. It really depends how far you're planning to carry it - if you have a drive and carry it a few steps into the house you'll be fine. If you have to carry it down the street and round the corner, it will be too heavy before long whatever you choose. If you have a pram that's very easy/quick to put up you can click it into, it doesn't really matter how far you have to walk. If you need to carry it up several flights of stairs, you might want a sling!
My opinion on your list:
Joie i-Level, Nuna Arra - think you're right about these.
Cloud Z - agree with you but it is a REALLY nice seat. I do personally find it a bit too expensive, given the amount of time you use it for and I wouldn't personally go for the Sirona Z, so the base is expensive for just using with this seat, but if you can afford it, it's absolutely fantastic and really well designed. (There is nothing wrong with the Sirona, I just prefer other seats).
Nuna Pipa Next - yes, nice, but sometimes the shape of the inserts is a bit too upright for a lot of babies, especially if you have sloping seats in your car.
Maxi Cosi Pebble Pro or Plus - love these seats, they are brilliant, the only downside is the very low height limit on them.
Maxi Cosi Tinca/Rock - I can't see much between these. Go for the Rock or look at the Cabriofix, which fulfils the cheap, light, safe criteria but doesn't have a height limit as it isn't i-size. The Maxi Cosi height limit of 75cm (all their i-size carriers) is really low and reached at an average of 10 months which is really borderline for moving into the next type of seat, as well as limiting the time you get to use the carry feature!
A couple of others I'd look at:
Britax Babysafe 2 i-size. Not as compatible with as many prams, but still nice. Has an internal angle adjuster and not as heavy as some more flashy ones.
Avionaut Pixel/Ultralite - unfortunately you can't buy direct in the UK, but you can get Venicci Ultralite, which is the same seat rebranded for Venicci pushchairs. Uses the standard Maxi Cosi adapters. The inserts in it give a lovely flat position for newborns, it's incredibly lightweight and it safety tests really well. The Avionaut (and I think Venicci) websites have a feature about the insert which works differently to most seats.
If you're planning to use without a base, the Maxi Cosi and Britax seats are by far the easiest to fit using seatbelt, the seatbelt guides on them are incredibly well designed and not fiddly. Other brands they often come across as a bit of an afterthought. Fine for occasional use but if it's your main mode of install you'll probably curse it.
Overall advice for the infant stage would be to save money on flashy features that you may not use, spend on features that are likely to make a significant difference, save money for the toddler stage which can be £££ and really matters if you get the wrong one!