The RT-AC68U is a pretty good consumer grade router out the box but there are a few things you need to do to tighten it up.
- Disable UPnP. This is perhaps the biggest hole it has as standard. If you really need port forwarding set it up manually
- Make sure you disable WPS, it is much easier to break into your WiFI with this enabled
- Make sure you change not only the WiFI network password but also the router login username and password
- Switch on AI Protection. While far form perfect it will give you an additional layer of browsing security as it runs its own dynamic site blacklist
- Add MAC address filtering so only whitelisted devices can join your network
- Make sure remote admin is switched off (if practical)
- It supports WPA2 Enterprise, there is more to set up but if you want better per user encryption it is worth it
Also I am not 100% sure why you switched to a class b network space. I can see not benefit to this for a home network.
Consider running https://www.grc.com/shieldsup against your set up or even better run nmap against your gateway to find out if there are any rouge ports open.
question: FireWall NAT Loopback, should I turn that off?
See here for details: Is NAT Loopback on my router a security problem?