If you have a great memory (you probably don't) and are 100% confident you won't forget the phrase then remembering the phrase is a nice option. Few downsides I see with it:

• What happens when you get in a car accident that damages your brain to the point that you can't remember the phrase?
• How do you pass your bitcoin on to your heirs?
• Unless you can remember 123 (or 122 if you use a custodian) you can't use multisig. Maybe this doesn't matter since you are just remembering your phrase? As someone else mentioned, someone could torture you and get you to give them the phrase. With a multi-sig they could torture you all they want and they can't get the bitcoin, assuming you don't have all your sigs in the same place at the same time.

You could also search a "border wallet" which I know sparrow supports.

Few other things:

• I think the best option is a multsig with metal backups, never keep enough sigs together to tx unless you 100% need to.
• Your HODL wallet should not have money going out once a month and you don't need to sign to send money to a wallet.
• As @DarthCoin has mentioned many times, you should have multiple wallets. If you are sending coins from your HODL wallet every month, you are doing it wrong, that should happen from a non-HODL wallet.

Please take the sats... if you can. In plain sight https://postimg.cc/6yRq074Y

I think if you want to memorize it, you can. I would not rely on it as a primary way to store your seed though. Memories are not perfect. What happens if you get older and your memory starts to go?

It is not impossible to steal something out of someones head. What do you think torture is?

Hardware wallet to deal with any management needs and steel for seed phrase backups. It will outlast you and your memory. It will outlast house fires, earth quakes and nuclear apocalypse.

My plan is foolproof.

I plan to have 14 kids, meet my children:

1. Flat smith
2. Crash smith
3. Ghost smith
4. Brink smith
5. Snack smith
6. Blink smith
7. Plump smith
8. Spook smith
9. Frost smith
10. Spin smith
11. Glove smith
12. Whiz smith
13. Grump smith
14. Twist smith

It's important to clarify the terminology you're using:

Cold storage refers safer storage which is typically access infrequently and at some inconvenience.

Multi-signature is the requirement of more than one signature to spend.

Air-gap means the hardware making signatures is not connected to the internet.

--

Not being signed in anywhere doesn't help you if you enter your seed phrase on a hacked machine.

I'm concerned that you're willing to trust a wallet provider... who are you talking about in that case, for example?

12 words might not be that hard, but songs are fun and meaningful and seed words are often nonsense.

You can put your seed phrase on a steel plate and still memorise it

Data Availability, Data Integrity, Data Confidentially.

Just think about what things could happen that would impact those three things.