In case of creating multi-share backup you need to understand what information an attacker will have when accessing one part of the backup.
There are no other special advantages of a 20-word backup over a 12-word backup, in both cases random entropy of 128 bits is used.
SLIP39 is generally much safer and it is supported by other HW devices like Keystone, and software wallets like Electrum.There are no other special advantages of a 20-word backup over a 12-word backup, in both cases random entropy of 128 bits is used.
Trezor developers wouldn't switch to SLIP39 if they didn't think it was not better than BIP39, and they invented both of them btw.
If you think that you can crack SLIP39 because of the ''flaws'' than contact Trezor devs and get bounty from them.
Good luck
