I've got LD2410B, which are mmWave, so they should detect heartbeat (and do), but not from more than 2-3m away.

Is the LD2410B sensing range limited by the PCB antenna?

I'm not sure, it always just made sense to me that the farther away someone is, the harder it'll be to detect their small movements.

Smallness detection depends on RF: higher frequency = shorter wavelength = more resolution for imaging.

Detection range without obstacles depends on transmit power and signal reception.

There are other models in the same family which might have more power/range.