Monero is assigned coin number 128: https://github.com/satoshilabs/slips/blob/master/slip-0044.md

As you say though, it's more complicated than just deriving at that path since you also need to use the sc_reduce32() function to end up with a ed25519 key.

There are various tools out there which will guide you through this process:
https://github.com/ph4r05/monero-agent/blob/master/PoC.md#trezor-seeds
https://github.com/LedgerHQ/app-monero/tree/master/tools/python
https://coinomi.github.io/tools/bip39/