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So if I have my seed, I can just reset the ledger and create a new seed, am I right in my understanding here? Or do I only need to add a 25th word to the seed, and then I have to wallets, one with 24 recovery and one with 25 recovery passphrase?

You already have a 24 seed, second wallet adds a word to that seed.
https://ledger.groovehq.com/knowledge_base/topics/advanced-passphrase-options