Hi Bored Git
One of my side gigs is engraving and milling (hoping to get some coin related boxes and inserts sorted at some point) and its something iv thought about doing something similar for a little while for personal use.
When i have asked around with people with a bit more experience the main bit of advice i got is to make sure your extraction is spot on (aim for overkill) as the carbon dust can play havoc with electronics/PCB's (or pay £££££ for a water jet mill). Im not sure whether this is a bit of an old wives tale as id not heard of a specific case of anything serious happening but the few people i asked all said the same thing.
One solution i came up with was to potentially use a wooden mould coating it in something like metasilicate but have never got further than the theoretical for this for silver, iv made up a few moulds that were going to be used for lead with this method but not sure if its transferable for SIlver. Using wood would definitely be a little kinder to learn with (and cheaper i would think).
If it is just for personal use another option could be use a standard rectangular/round etc mould and the machine the bar/round after, you should be able to get plenty of detail etc but it may not be the look your after. This will be the route i go down i think as the art of pouring is not something i have remotely enough time to master at the moment. Definitely not commercially viable though.
If you do manage to get set up id be really interested to hear how you get on, would definitely be cool to be able to make your own custom bars