So, why settle for a fraction of security if you can keep full security?
I have looked into the shamir schemes but unfortunately this is not a solution at the moment as the scheme is not compatible to split BIP39 Mnemonics into a human readable format (Writing down long strings of HEX by hand is not the best idea), It would need to be compatible with my Ledger. They are supposedly working on a SLIP39 implementation, like Trezor just released. This would be the ideal solution as it retains 128-bits of security even if 1 share is compromised.
I already have my unencrypted 24-words in 3 completely separate locations. One of these copies is with a trusted 3rd party. Im not worried about the person who is caring for it compromising it, more from other people who may come across it (Thieves, Burglars, Snoopy Guests, Guests Children etc.) or if I was to leave a copy in a safety deposit box that it couldn't be compromised by malicious staff or security camera immediately. Id rather have some security from this then none. Adding a passphrase would only be an option if it was high entropy (over 85bits, anything under this would have less security then the missing 8 words), and then I would have the same issue of having to store the passphrase somewhere safe.
I really appreciate the responses
There is another technique that could satisfy your requirements, i.e. preserve security strength and "human readable format": exclusive OR (XOR)/Vernam cipher (a.k.a. One Time Pad). Your secret "BIP-39 phrase" is encrypted into and two (or more) complementary ones. One could be stored in safety deposit box and the other in a separate location. Both of them would be needed to decrypt the 24 words. There is more information with examples at "github.com/GregTonoski/BIP39-XOR".