This is the other way to get wifi on older machines... through modem emulation https://subethasoftware.com/2018/02/28/wire-u … modem-firmware/
The most plug and play way to do it through a network card is to use a WAP standing for Wireless Access Point... not Wet anything... but since those waned in popularity after b standard, old and cheap ones are likely to be b only, and ones I used back in the day seemed to be buggy, did things like lock up and overheat. More recently there were some made as wifi adapters for consoles with a RJ45 LAN port... but unless you snag one for cheap in a thrift etc, they tend to fetch some tens of dollars as the console guys still want them.
That leaves us with as noted, an abundance of discarded routers. Many you don't even need to mess with much just set them to bridge mode. I have noticed though that the built into a wallwart type wifi extenders quite often have bridge mode as it's their default operating mode, and those are kinda compact, and only have the one LAN port... so one per device or use a hub/switch and serve a retro cluster. But at that point the regular old 4 or 8 port dealies are looking better.
Personally, I have been finding that powerline internet adapters picked up cheap are serving me well for retro boxes, some of the ease of wifi, no running cables apart from adapter to NIC, and router to adapter at the other end, and cross house connections doable. They need setting up with a newer machine initially, but then will work after power outs fine, guess they have flash memory for settings.
Unicorn herding operations are proceeding, but all the totes of hens teeth and barrels of rocking horse poop give them plenty of hiding spots.