Would be nice if the Nano were available in a castellated single-sided board, but even the Nano Every and Nano 33 series of boards do not bring the USB connections out to the castellated connections. Probably not a lot of market for intra-board USB communications on a processor that supports I2C, since the USB does need a hub.
In terms of wanting to use a common crystal oscillator for multiple devices, would your application work with the atmega328 running at 12MHz, so it could share the oscillator with the CH340?