Like you I don't want to waste things and I only started realizing that every option that I choose adds both the access points and the switches.
I don't "maintain" my routers since nowadays they can upload the latest firmware on their own, and they take their IP address from the modem too.
Can't argue with you that a dumb switch is the easiest method, which is why originally I was only thinking about re-use as a switch but then I realized while looking at all the responses that EVERY option adds two things anyway [1] three or four extra ports [2] two Wi-Fi access points (one 2.4GHz and one 5GHz)
If anything, I have more appreciation now than before I opened this topic that a router is a powerful set of switches & access points no matter how you configure it for a typical home environment.
I only recently realized another use can be to augment a PC's weak Wi-Fi NIC by connecting the router to the Ethernet port and using it for three different purposes all at the same time (which is amazing).
[1] It starts as a wireless client bridge connected to any AP in the home [2] For free it adds a few more ports at that PC if you need more Ethernet [3] And for free it adds two Wi-Fi access points at the location of the PCI don't see any disadvantage of this other than it uses up the one Ethernet port most PCs have but if you're not using it, then that's not a drawback.
On a humorous sidenote, I observe that going wireless has produced more wires than I had before I only had long ago wired devices. :->
I've run cabling too but I prefer for this re-use not to, so the only cabling will either be from the new home router to the old re-used router (to get more ports mostly as I don't need more access points in the office) or from a PC ethernet port to the old re-used router (to extend its range and to add more ethernet ports to the pc and to add two more access points at the location of the pc).
Practically, that option seems to be the best but it would likely be only useful for a static desktop and not useful for a laptop which moves around.
One trick I've used in the past to get signal from one end of the house to the far end is to drill a hole in the outside wall where the modem cable runs into the house and run the cat5 cabling from the home router in the office out that hole and then around the outside perimeter of the house and then either bring the signal back inside the house at the other end by drilling another hole or without drilling the hole using an access point just outside the window pointing back into the house.
I'd use the router if I could but it has to be outdoor equipment for that.
With all this advice, I'm now leaning toward using the extra router to augment a desktop pc's existing Wi-Fi NIC because the extra router might be more powerful than the PC's internal Wi-Fi NIC, and it also adds extra ethernet ports to the PC and it also adds two additional access points to the PC which cellphones can use.
This seems like a perfect solution for a desktop PC far from the location of the main home router which is usually located near where the modem is.
The only requirement is the desktop PC needs to have an empty RJ45 port.
Once I set up a router, I don't look at it for years, and because of that I often tape a written description of the login details because I don't touch it for another five years and then I forget what the login credentials are.