Just a LoRa gateway without the WAN stack? There are usb serial devices for 20-30 pounds, you can use a raspberry pi with them for a cheap gateway. We’ve used some for development and testing. https://www.st.com/en/evaluation-tools/b-l072z-lrwan1.html
There’s a few layers to this. You’re completely correct that you can use those boards for LoRa without LoRaWAN. I think what OP is looking for, though, is cheap LoRaWAN gateways that aren’t tied to The Things Network. Can’t use the SX1276 for that, since it can only time one receive frequency at a time.
There’s cheap gateways available that are hard-coded to talk to TTN, and there’s expensive gateways available that let you run a private WAN. There just doesn’t seem to be cheap gateways available to run a private WAN.
I have read rumors of alternate firmware to allow that hardware to work outside TTN but haven’t chased it down myself, so mine is running as a TTN node right now.
You’re looking at around $150 for an 8-channel base node. The single channel at a time nodes (like the SX1276 in the one you linked) “work” but with significant limitations compared to the $150-500 8-channel gateways.
There’s cheap gateways available that are hard-coded to talk to TTN, and there’s expensive gateways available that let you run a private WAN. There just doesn’t seem to be cheap gateways available to run a private WAN.