The suggestion to use iCloud is a good one. I don't know what SpiderOak offers or what security they claim, but if you use iCloud and 2FA, the data will be about as safe as you can make it. If you want more, encrypt it before you send it up to iCloud, but Apple's security is among the best. I'd rather have Apple, with their stated attitude toward privacy, holding my data that some other company about whom all I know is what the Internet tells me.
The freezer suggestion is not one I personally would take. The cold temperatures will cause the USB drive to shrink in size slightly, but the various components may well contact at different rates, leading to stresses on the solder connections of the surface components. Add in the little bit of moisture from the air trapped in the ziplock bag and you have a terrific potential to have the USB drive shrink but the water condensing and expanding as it freezes. Things could go wonky pretty quickly. The refrigerator section, maybe in the vegetable bin, might be slightly better and just as protective of fire. It should be above freezing, so the contraction/expansion will be less in both directions, reducing the overall stress.
I guess the "castigation" I would offer is that unless your flat gets broken into frequently, you really don't need to be that paranoid about credit card information. I had suggested 1Password and you thought its recommendation about the Emergency kit was humorous, but if you think about what they are saying, the Emergency Kit is
not a list of your passwords or files, but an emergency kit for the Master Password to enable you to decrypt the 1Password where the passwords/files are encrypted if you forget the master password. So what you might carry around would NOT be the data itself, but an emergency key to it.
Frankly, what I would do is get 1Password, put your sensitive stuff in it, then print off the Emergency kit and put THAT in the freezer in a zip lock bag. Now 1Password can sync to your smartphone so you have the data with you, AND on the Mac, both encrypted and secure so that even if Burglar Bill gets in and takes the MBP, the sensitive data is locked away from his prying eyes. You need one password to decrypt it all when you want it and that one password is safely secured in your mind with a backup in the freezer below the frozen veggies. Remember, if you lose that Master Password nobody, including the good folks at 1Password, can open that file for you. The Emergency Kit is like a spare emergency key to your flat that you leave with a trusted neighbor, just in case you lose your keys. Here are the suggestions 1Password makes in the discussion of the Emergency Kit:
Follow these tips to prepare your Emergency Kit and store it safely:
Print a copy to keep in a safe deposit box or wherever you keep your passport or birth certificate.
Write your Master Password in at least one printed copy of your Emergency Kit.
Keep a copy on a USB drive to make sure a digital copy is outside of the devices you use with 1Password.
Give a copy to someone you trust, like your spouse or someone in your will.
Those are options, so you pick the one you want. Personally, I would not use the USB stick because of their unreliability and relatively high failure rate. Paper won't fail, so printing and hiding the emergency key to 1Password seems more logical to me. You could put it in an envelop and give it to a trusted neighbor and ask them to hold it for you, or tuck it into a book in your library, or mail to a relative, or just about anything. Keeping it off-site will avoid losing in a fire or burglary.
Or, if you are comfortable, you can keep doing what you are doing but to me what you are doing is the more risky approach. Lose that USB stick and your data is gone. Have it stolen and your life is exposed to the thief. Have it fail and all that data is gone. It's your choice.