With home routers, the focus is not on the hardware, but the operating system.  And then what hardware at what price point fits that OS.  So OPNSense and pfsense are two top notch choices for an initial firewall OS.  Then send that to a DD-WRT or Open-WRT wireless router behind the firewall.  

Internet → Firewall → Computer with Ethernet port
or
Internet → Firewall → Wifi Router → phone or laptop wifi

You could go direct to Wifi Router and skip the firewall, but these are less resilient security flaw wise and lack the firepower for a VPN.  Given that Wikileaks shows the CIA loves to hack home routers, I love the advanced security of OPN and pfSense.  We offer consults to setup anything you want, guide you through each step of the process, and offer continued support if you face any difficulty.  Not only can we stop DNS or VPN leaks, but additionally we can help you spoof the MAC address on the firewall, so that when you move from home to home, the new location won’t know it’s you.  All of this for a rock bottom rate of just $50.  You will not find more knowledgeable cybersecurity experts to work at third world peasant rates for both devices configured plus a consult. 

 I use ASus AC-68U with Asus Merlin. If read I cannot use OpenWRT with it due to broadcom, ist it right?
I used DD-WRT before and it was very unstable, so I Stuck to merlin. The problem is that theres no PF firewall on it.
Whats your recommendation to me? To try dd-wrt again on asus, or keep on asus merlin?  

 You should checkout the IPFire project! Opensource, very active community.