"premined" Crypto-assets on the bitcoin blockchain backed by physical goods like silver already exist now:
No, that is something you can purchase with BTC, and a massive amount of trust has to be put in their partners to ship the metals and not turn it into some type of fractional reserve system. Plus I do have an issue with the fact that they say they are willing to sell palladium for 0.0% over spot, are not transparent about shipping costs and do not seem to make a healthy enough profit to do what they seem to be doing long-term.
Regarding the "massive amount of trust that has to be put into their partners to ship the metals and not turn into some type of fractional reserve system"
Precious metals are stored bonded, insured and extensively guarded at segregated accounts (meaning you could withdraw even if the company dissolved) at International Depository Services of Delaware. Holdings are audited daily and you yourself may request an audit. Individual proof of custody is issued for each and every unit added to inventory. The whole setup is fairly transparent compared to many "digital gold" offerings on the market today. Delivery is via the custodian if it's requested later or directly via the dealer-partner if delivery is requested immediately. I found it fairly simple to find delivery costs.
As for why Palladium is being sold for 0.0% over spot, that's because it's not being directly sold by serica, it's a user on the platform who has listed that for sale. They could well sell it for 50% under spot if they wanted (on site or via the decentralized exchange.. they could swap it for any other asset there too). It's kind of like amazon, where multiple sellers can offer the same product at whatever price they choose and a user can choose who to purchase from. . If you play like to play with both bitcoin and precious metals and perhaps want to merge between the two often this may well appeal to you. If you just want the physical metals straight away there is no need for this, same if you want futures, derivatives anything where you don't really truly care if you what you are trading is redeemable for physical..
It's easy to pick holes, harder to suggest a stronger approach. We are dealing with tangible goods- there is only trust minimization, no complete trustless approach, it's just about narrowing it down to parameters you can be reasonably comfortable with. I have to say I would be more confident storing PM"s with this service via the approach you suggested, which amounts to "occasionally taking pictures of the physical assets with a webcam" and "Dropping the silver rounds on a counter so people can hear the silver for themselves"