I'll work with what I have and modify my setup as suggested.
Looking at the list of your client devices I don't see any IoT separation requirements, so what I would do is the following:
1. ISP modem in bridge mode (if possible), or with RT-AC1900P IP address in DMZ (if double NAT is the only option)
2. RT-AC1900P as a Main Router, running Asuswrt-Merlin firmware*
3. RT-N66U in Access Point mode with WiFi disabled and antennas removed (as Gigabit Switch for extra wired connections )
* On RT-AC1900P you have the choice of:
- AiProtection (simple packet inspection engine)
- Adaptive QoS (mainly to prevent bufferbloat in your case)
- Parental Controls (using TrendMicro engine or DNSFilter option)
- DNS-over-TLS (for added DNS security)
- Custom Scripts** (USB drive is needed)
** From custom scripts I would eventually run the following:
- Disk Check utility (to check the health of the USB drive)
- Swap File (required by Diversion and Skynet scripts)
- Diversion (DNS based blocker), not for ad-blocking though, but with EasyList trackers blocking, custom lists option available
- Skynet (IP based blocker), default configuration is good, options for custom configuration (like countries blocking, etc.)
- scMerlin (automates some tasks with a simple menu)
I would also set Phishing/Malware blocking DNS like CleanBrowsing DNS with Security Filter option (DNS-over-TLS is supported). It's not the fastest DNS service available, but trusted, effective, easy to use and free. For ad-blocking uBlock Origin browser plugin (or similar) work best, IMHO. If you need VPN on some computers, run the VPN client on the computers (your router's CPU will limit the OpenVPN connections to about 50-60Mbps).
Having DNS filtering, AiProtection, DNS blocking and IP blocking should provide more than enough security for a home network. You may see some maximum Internet speed penalty because of everything running on the router, but you can't push full Gigabit over WiFi anyway and game consoles don't need Gigabit connection either.
The above are suggestions only, there are many options to experiment with. Nothing is the best possible option, but you can have a pretty good setup with zero investments, using the existing hardware. All the information you need around Asuswrt-Merlin firmware options is available here on SNB and fellow SNB forum members are always ready to help.
Is the RT-N66U still supported with recent router updates?
John's Fork only, but I don't think this RT-N66U has to be used as a router in this setup.