It's incredibly risky. I'd argue that having your back up in the same physical location as your wallet is hardly a back up at all. The only thing it fully protects against is failure of your wallet, and does not fully protect again theft or any natural disaster, be it fire, flood, explosions, hurricanes, what have you.

I don't disagree about a single point of failure, but I prefer not to use multi-sig set ups (unless required between multiple people) due to their inefficiency, both when creating and signing the transaction, and in terms of the size of the transaction and the fees you pay. If you are worried about a back up of your seed phrase being a single point of failure, then I would prefer to add a long and complex passphrase backed up separately, or encrypt my seed phrase before backing it up and again, back up the decryption key separately.

Although metal based back ups are good, I wouldn't buy a Cryptosteel Capsule. Although it performed fairly well under stress testing (https://jlopp.github.io/metal-bitcoin-storage-reviews/), the price tag of $80 is completely unnecessary. You can walk in to any hardware store and buy a piece of stainless steel to etch your words on yourself for under $10, with the added advantage that you don't have to give your name and address to a company which only manufactures crypto-related products, since the recent Ledger hack has shown how dangerous that can be. Note that the sister product, the Cryptosteel Cassette, performed poorly under stress testing.

I would say a word of caution here: There are countless posts I have responded to on this forum from users who came up with their own "ingenious" way of storing/backing up their seed phrases, and two or three years later cannot remember their process and are unable to access their coins. It is better to use tried and tested industry standards such as proper encryption than it is to come up with your own system which will be both less secure and harder to recover.