python-mnemonic VS bip39

Yes. BIP39 defines a check word to ensure the previous words are correct. You can read the spec here: https://github.com/bitcoin/bips/blob/master/bip-0039.mediawiki

ColdCard follows the BIP-39 standard for encoding the wallet's seed as a series of 24 words (a.k.a. "seed menmonic", sometimes called a recovery phrase). If you followed the instructions you were given when initializing it and you've stored your seed mnemonic safely in the real world (not digitally where it can be more easily discovered), you're fine. In the event you lose or damage your ColdCard, you can initialize a new wallet--ANY wallet that follows the BIP-39 standard, which is pretty universal these days--and the wallet will regenerate the same private keysthat your original ColdCard did, in the same order.

... and BIP-39

Look-up "BIP32", "BIP39" and "BIP44", those will point you to documents describing precisely how seed phrases are converted into crypto keys.

i think bip39 is what you want to look up.

Not correct See this: https://github.com/bitcoin/bips/blob/master/bip-0039.mediawiki

If you are going to take this route, I'd recommend using Shamir Secret Sharing to split your key into shares, of which you can choose some number required to reconstruct the original key (like 3 shares, any 2 can recover the key). Keep the shares in disparate locations, maybe one on your encrypted usb, one in a safe, one in a bank lockbox, one with a trusted person...

This is not good. By finding just 1 share a potential attacker has reduced the time needed to crack your phrase thorough brute force 96 sexdecillion times. Check out this amazing video about the Shamir Secret Sharing scheme which allows secrets to be split up in such a way that no useful information is revealed until the required number of shares are put together. Personally I used this repo to generate SSS shares.