You could consider taking the Electrum route and sending all of your BTC inside an Electrum wallet and memorize the seed so a file doesn't exist, however, how safe is this? Keeping all of your current and future addresses on there doesn't seem too sound. It is possible that if someone subtracts the necessary info key derivation could be exploited.
So wallet.dat is the safest, yet, the most annoying to keep safe, as you need to keep a physical medium to store it offline, and you need the synced node to transact as well.
Actually I favor keeping the seed or the private key printed or noted in a book, whether buried somewhere or hidden in plain sight.
wallet.dat is harder to store because all means can be either hacked, either have faster decay rate than anything else, whether we talk about USB sticks, HDDs or CD/DVDs.
I clearly don't advise memorizing the seed since there are many things that can make you completely forget in a couple of years what the seed was.
If the amounts are indeed significant, I'd also try to not show that off.
Because if unwanted ones find out, whether the government, whether simple thieves could come after you and no digital protection can overcome
the 5$ wrench.