Anyone has done mesh WiFi (ideally triband) using off the shelf parts and Linux?

I have an Orbi AX system which works reliably, but now I want to upgrade the radio to WiFi 7 and that means I need to upgrade all the hardware.

Hoping to move to using off the shelf parts so in the future I can just change the radio (ideally bunch of USB sticks).

I understand this is not strictly just the router. I can (and used to have) a router as separate device, but any mesh WiFi right now that I can find need a pricy router that acts as the coordinator, essentially negates the economic benefits.

That's a bigger can of worms than you might expect. Most consumer WiFi chips only barely support AP mode, and I'm not aware of any that can do multiple bands simultaneously. You'd probably need 4 adapters on the repeater for triband. One to connect upstream, one for each downstream band. Three instances of hostapd all configured with the same SSID and auth for each downstream interface.

Then there's the roaming issue. This is largely what the commercial "mesh" systems try to solve: deciding / helping inform when clients should switch APs. There are many solutions and none of them are without issues, including the commercial ones. Here's a starting point: https://openwrt.org/docs/guide-user/network/wifi/roaming

Openwrt guys were cooking up a wifi 7 router I think. Think that’s best bet but Not super close to it though