If you were to store a significant amount of BTC today and not touch it for the next 10+ years, would you trust a deeply encrypted legacy wallet.dat backed up on multiple offline drives, or a standard BIP39 seed phrase stamped on metal?
A wallet.dat file commonly refers to a file based wallet of Bitcoin's standard client software Bitcoin Core and its forks. Those usually rely on file based backups, because by default Core doesn't use a mnemonic recovery words backup scheme.
You could setup a descriptor wallet.dat from a standard BIP39 mnemonic seed phrase but Core doesn't do this on its own. You could of course export modern HD descriptors in text form from a descriptor wallet.dat in a secure preferably offline environment. You should know what you're doing and how to do it safely!If all your redundant file based backups are in one place, this place becomes and is a single point of failure. (Like a fire could destroy all your redundant backups and then you lost them all.)
Any long-term self-custody hodling needs a proper loss risk assessment and suitable strategies to remedy the most relevant loss risks. I would suggest to not try to overdo it. Something too complicated to mitigate too much can become a loss risk by itself.
Same applies to self-custody using BIP39 mnemonic recovery words. You should have redundant backups, I use paper and metal stamped backups. Do a proper loss risk assessment here, too!
People make errors because they don't think it through and in many cases also lack the knowledge about how Bitcoins and wallets work, about security, about safe environments. They over-estimate their abilities to memorize things over longer time periods. They document poorly in many cases.
I don't want to hide that good and extensive documentation can become a risk, too. But the lack of it
IS definitely a loss risk or rather a recipe to loose coins.
You may want to take a look at different approaches to secure self-custody and related topics (no significance of listing order):
e.g.
https://glacierprotocol.org/https://www.smartcustody.com/ --- IIRC, they elaborate quite a bit on risk assessment
[Guide] Secure air-gapped crypto wallet storage method --- dug up an old guide, maybe worth a look in addition to my previous poster's links
https://blog.lopp.net/a-treatise-on-bitcoin-seed-backup-device-design/Securing Your Seed Phrase with WashersPost edit:
Very briefly, what do I use?
- I think I made a good enough loss risk assessment and based my backup strategy on it, trying to avoid too complicated setups.
- I pay attention to sufficient and good documentation. I don't leave docs in plain sight at home (a burglar could find it with enough time, but that's not a likely situation). An encrypted copy of my documentation is at a redundant other location, same for my off-site backups. The digital documentation part is solely on an always offline computer.
- I don't use Windows for my crypto coin stuff, Linux instead. I don't use my regular daily computer for my crypto wallets with the exception of watch-only wallets.
- I use decent and trustworthy open-source hardware wallets (I don't ever use Ledger crap).
- I have redundant paper and stamped metal washers mnemonic seed words backups and currently also use additional mnemonic passphrase extensions that are backed up separately from BIP39 or Electrum seed words to avoid loss by exposure of the mnemonic recovery words alone.
