Here is the issue with modularity like you are planning. If a cable is loose because something bumped into it you now have a problem to chase. Even worse you now have the potential for a power cable to become completely unplugged while the host is still up and running. Having two power supplies also creates a potential for a ground loop through the data cables if things are not made correctly.
Some people have been able to use USB drives connected to a computer to create a NAS but they are also the ones hitting forums because they have issues. What you are trying to do isn’t much different. If you want to keep your hardware you would be much better off getting a case that can hold all your drives. A used server chassis would be ideal if you use a SAS card. But if you are going to upgrade anyway maybe it’s a time to make plans to make the switch sooner than later.
Do some commercial solutions for this exist that are safe, yes they also live in racks and are effectively ground connected with the cabling protected/much less likely to be bumped into.
Many open source operating systems exist that can turn a computer with multiple NIC’s into a router or can be used in place of a hardware router OS. https://distrowatch.com/search.php?ostype=All&category=Firewall&origin=All&basedon=All¬basedon=None&desktop=No+desktop&architecture=All&package=All&rolling=All&isosize=All&netinstall=All&language=All&defaultinit=All&status=Active#simpleresults is a search on distrowatch.com that gives you a petty good list to get started.
I personally use OpnSense with a Supermicro motherboard a Xeon E3-1226 v3, and 16GB of RAM. It was all used server equipment bought on Ebay. I run Caddy, an ACME client, Intrusion Detection, Chrony, UnboundDNS, Wireguard as a VPN endpoint, and Wireguard as a client for IPv6 connectivity through Route64 because my ISP only has an IPv4 stack. For WiFi access I’m running a couple TP-Link Omada EAP-650’s with the OC200 controller using POE so I can place them in ideal locations.
Will a firewall prevent issues if the Asus devices have some sort of Spyware on them. It can but not by default. Generally firewalls are configured to stop anything coming in and let anything out. Since the RT-AX3000’s are on your internal network by default they can send data out. Something like Intrusion Detection can watch for bad things running on your network and help but you would have to set static IP’s on each one and null route them. You could also flash them to an open source firmware if you are worried but is a personal decision.
I avoid two things in networking, router modem combo devices and really cheap routers or access points. Honestly you should ask, “Why is this so cheap?” Then look at the reviews for those super cheap Chinese android tablets and computers and you should begin to understand my reasoning why.
Also used commercial grade hardware on Ebay is a great place to get a steal if you are building a homelab. Most of the time this stuff is pulled because it no longer is fast enough for a server farm and functionally obsolete. The firmware will generally be very stable and well tested. I’m running a 10Gbps fiber backbone for my network that connects my router, server, 48port ethernet switch (using 2 DAC cables), and desktop computer together.
I have a 1Gbps fiber connection and speedtest at 950Mbps while everything is up and running. The Ethernet connection at 1000Mbps is the limiting factor. A speedtest from my cell phone (S26) over WiFi I test at 680Mbps. My testing internally from my desktop to my server using openspeedtest runs around 8000Mbps.