I have the Muum/Matrix Light 2 combination for my 5.5 month old.
I use it every day for walking and train journeys and also several times a week in my mum's tiny car (3 door Citroën C1).
The original Matrix Light got bad reviews however the Matrix Light 2 scores well on Which. Their only complaint is that the Isofix base is tricky but I just use seat belts which work fine.
As mentioned above the car seat is heavy but it's perfectly doable to get it in and out the car. my DS is over 18lb and I can still get it into the back of the car in lie flat or sitting up mode.
I tend to use the lie flat option in the car while I can since it just looks much more comfortable to me, although he is at the age where he likes to look out the window so I probably won't get away with that much longer! The lie flat position is an option to 10kg and the upright position is suitable to 13kg so you get a bit more use out of it that a maxicosi or similar small seat.
In addition to the 2 car seat positions there are a couple of reclined positions you can use when the Matrix is on the pushchair chassis. All of my baby groups buddies have had to move on from their carrycots as their babies don't want to lie flat all the time so this feature really extends the life of the carrycot function.
I bought a Moom footmuff which is advertised for smaller car seats but fits the Matrix Light 2 as well. If you decide on this seat I'd highly recommend this extra expense, it's very comfy and waterproof and DS loves it.
I considered the three pushchairs you mention (Rider/Trider/Muum) and decided on the Muum in the end. I prefer the standard 4 wheel set up and I really like the way it folds, its so straightforward compared to other pushchairs we looked at, and it fits in the tiniest boot (albiet by popping one of the wheels of but this takes seconds).
It can be folded with the pushchair seat on but I've not yet had to use this as DS is still in the Matrix. The seat unit on the Muum is larger
than the Rider/Trider and while the other seats do look a bit more stylish the Muum unit should last me well into toddlerhood!
Sorry for the essay but I remember not being able to find much info on these options when I was looking so wanted to help as much as possible 