I used some Decawave kits ~a decade ago and they worked pretty well... however if you want to DIY something as usable as Airtags I think the main struggle would

I used some Decawave kits ~a decade ago and they worked pretty well... however if you want to DIY something as usable as Airtags I think the main struggle would be getting the package compact and with low power consumption (unless you want to track large things and are fine with a chonky battery)