Never gonna use an SD card for this.
How is an SD card worse than a USB drive? They both use the same flash drive technology. You could as well buy a whole SSD drive, but why waste money.
It actually might be a good idea to keep a card hidden somewhere in your wallet. This way you'll have access to money in case of emergency like a house fire. Of course this is an option for people heavily invested in crypto, who don't have much fiat.
It doesn't matter how much crypto or fiat one has, offline storage is the most secure.
How do you encrypt an SD to carry it around with you? If you lose it, you will probably need to change the keys anyway.. and the chance to lose a wallet is too high..
There are many things you can do to secure the card, one of them is a simple archive with a password. Another is hiding the real file among a number of fake ones. All of these things will buy you time if the card gets stolen.
Nobody forces you to have all your money on that card and yes, you will probably have to move cash from the wallet, but it doesn't seem like much of a problem. The person, who steals your card will still need the password (or passwords, if you use encryption or protected archive) to take your money, while you'll just get home and move it to a new address in less than an hour.
