That's addressed in the second link OP shared. They use m/0/0 for the first address, and simply increment by 1 from there.
They claim that it is also compatible with Trezor, Ledger and KeepKey
extended public keys... so it doesn't sound like they're doing anything out of the ordinary as far as derivation paths are concerned.
Understood. So I will create a cold wallet only for Bitwage and later on I will "mix" the funds and transfer them to a private wallet.
Yes, that would be a good strategy... a wallet
only for Bitwage... and then later you can utilise Mixers or CoinJoin etc, to be able to get the funds into a private wallet in a way that should effectively obscure the links between all the funds (at least from the average person).