For roughly the same price there are some DVR boards that could be converted to SDR operation once they're hacked. They already use embedded Linux although no source is provided.
Yup, I know the HackRF. Being too old to write Santa a letter I'm already saving money for that gift:).
Anyway my point was that those cheapo DVR boards are just asking to be repurposed for other tasks, SDR is just one of them, that chipset specs are interesting.
https://www.burglaryalarmsystem.com/pdf/Hi3520.pdf
Here are some firmware files for a few models, hopefully compatible with that one. Binwalk-ing them shows they're normal ARM Linux images.
https://www.aliexpress.com/item/HackRF-One-1MHz-to-6GHz-SDR-...