With a 120-square-meter apartment and dead zones, a mesh system is probably the better solution for you, to be honest. A single powerful router wouldn't likely fix the problems in your bedroom and kitchen completely, because range is just limited - no matter how good the router is. Mesh works differently: you place several devices in different rooms that communicate with each other and create a connected network. You'll really notice that when you're browsing, streaming, or gaming - the connection stays stable as you move between rooms.
About the Fritzbox 7690: it's actually a mesh system (or at least designed to communicate with other Fritzbox devices), not just a single router. It costs somewhere around 150 - 200 euros, depending on where you buy it. Eero systems start at similar prices, but you often need two or three devices for 120 square meters, which gets more expensive overall. If you already have a Fritzbox and you're happy with the ecosystem, the 7690 is a safe choice - you already know how to use it and compatibility isn't an issue.
A better single router might save you 50 euros, but the dead zones would probably still be there. Mesh costs a bit more, but it really solves the problem you're describing. If money's a concern, you could try a better single router first and upgrade later - but honestly, I'd go straight for mesh if your apartment is that large.