Child car seat / Driving safety

Hello,

I brought a car seat for my 5.6 yrs old from Amazon Germany however I face the below issues

1> It is not an isofix seat
2> The cushioning is not too good. The child gets tired in long journeys.

I visited few shops in Belgium (Cora / Carrefour) but I did not find many options.
The one's I liked (company safety) did not have good cushioning.

I checked some options on Amazon.de and the brand kinderkraft looks good.
But I don't want to buy a product which is uncomfortable or unsafe.

Can someone please guide me, which is the best child seat in terms of safety and comfort for my kid.
I do not mind spending because is nothing more important then my child's safety.

Already, I thank you all for the inputs.

Thanks,
Ro.