For, e.g, handing out a dozen of phones to a family,
Is there a way to backup the whole OS with all its profiles ? As the install jumps from the bootloader to the OS installed, without some recovery like TWRP in between. I don't mind temporarily unlocking the bootloader

    graphenediscoverer4
    Unfortunately not.
    Seed Vault is of very little use or is unreliable, which is why the GOS developers have long been looking for a proper alternative or will develop one.
    It's not all that simple ...

    Workaround: Use apps, which support saving/export your data in a specific directory on the device.
    This way you can copy them on a regular basis e.g. every day/friday to a connected USB card reader with a SanDisk card or send this data yourself via an end-to-end encrypted mail service like Tuta.com.

    Bitwarden is a great platform-independent open source password vault, personal use for free (with 2FA for master password included!).
    And for managing E-Mail, Calendar, Contacts use a serious trust-worthy service like https://tuta.com.

    BTW: it would be nice to take a little more care when typing on a broken (?) keyboard and always check your text again after typing before saving it.
    I don't know “nandroid” and “gOS” doesn't exist either. A little respect for names would be appropriate.

    But if you are visually impaired and therefore didn't notice, then sorry and forget my last paragraph.