Of course, you would still have the secure erase problem if you leaked the key, because you can no longer consider the cryptodisk secure.

I get that there is variation between hardware implementations. For that matter, same could be said for magnetic HDDs. But at the end of the day, I doubt all the sweatshops in China that make 1001 novelty flash drives have an army of wild geniuses work for them that come up with a completely different design every day. They're also extremely allergic to adding any more redundancy than strictly necessary (sometimes not). So there probably are a couple of "typical" designs in circulation, and it would suffice to have a method for those, with the caveat that if you have an atypical device it might not work.

For example, I would expect that at some point the meager wear leveling of a cheap drive gets saturated with writes and very little, if any, of the data survives.

If you use secure erase you don't ever actually get the key - its just kept on the disk. The risk (which is relatively low unless you are talking about multi-million dollar secrets) is that secure erase has been backdoored or improperly implemented (and this is not theoretical - some early disks had this issue). The whole point of using Secure Erase is it works fine with SSD's and does not cause write amplification, while providing FDE in case of erase.

What I'm saying is, the logic in this answer appears to be:

Encrypt the SSD, and then it doesn't matter if people can recover the data because they wouldn't be able to decrypt it

However, the flaw in this logic appears to be:

If your encryption key happens to leak, you can no longer consider the encryption safe, because now anybody could have the key, so you would need to securely erase the encrypted data to prevent someone in the possession of the leaked key from recovering and decrypting the data

It's all defense in depth. Do nothing, and the adversary just has to get their hands on your drive. Erase the data, and the adversary has to get their hands on your drive and find the data on a discarded but not erased block. Encrypt and erase the data, and the adversary has to get their hands on your drive, find the data on a discarded but not erased block, and get access to the key from a second device. Arrange to erase that device in addition to the above, and .... There is no ‘safe’; there is only increasing the cost of defeating your defenses.

It seems like "encryption" doesn't really answer my question in that case.

As I said, to answer your question as written would require knowledge of your hardware—not because every USB stick is manufactured by a maverick genius, but because there are no standards for secure erase in common use across SSDs and USB sticks. The bargain basement devices are unlikely to let you do it at all; your data may live in retired cells until the NAND charge leaks out and unless you have the equipment to read those cells directly, you'll never know. ‘Encryption’ was not intended as an answer to your question; it was a description of what I do knowing that the answer to your question rounds down to ‘good luck’.