Re. this question on Super user, I can't use iPad + Luna as primary monitor and FileVault simultaneously. My question is then, is there another way to conveniently encrypt the Document folder on the Mac? In other words, a way to replace FileVault as seamlessly as possible, targeting only the normal user documents?
Depending on your definition of "conveniently." You could make an encrypted .DMG and store your sensitive docs in the disk image.
Once mounted macOS sees the mounted image as just another volume, the only part requiring a password is when mounting the image.
If you created it as a .sparseimage it should expand as you need it to without having to create a large image that would take into account future usage.
VeraCrypt is an open-source, code-audited encryption tool for most platforms, including MacOS. It allows the creation of encrypted "containers", which may be what you need for just encrypting the Document folder.
With reference to your requirements of "conveniently and seamlessly as possible", bear in mind that other methods are likely to incur speed penalties in Disk operations. (FileVault2 itself has some disk speed reduction, and FV2 and Mac hardware are designed to help each other perform the task).