If it makes you more comfortable, you can store your passphrase separately from the rest of your seed.
You
should store your passphrase separately from the rest of your seed. Storing them together is almost pointless.
Ledger Nano uses the derivational path that you posted above (m/84'/0'/0'/0) for native segwit address generation.
This is true, provided that the address OP has is a receiving address (and not a change address) in the first bitcoin account they created using their passphrased wallet. If it is a change address, you would need to change to m/84'/0'/0'/1, and if it is not the first account, you would need to change to m/84'/0'/1'/0. If OP does not know or remember, then they are going to have to check all these paths and more.
Check
btcrecover. It's a tool made for these occations.
Agreed. This will be a much faster way of checking. On an offline computer, feed it your seed phrase, a list of possible passphrases and their variations, and the address you know (and widen the derivation path limit if you don't know where the address came from).