The whole post is really big and many won't go through the full post at all. Also, reading makes it hard to understand. So here's a picture I created to explain in as a simple visual learning thing. I have created this maybe that's why it's not that much complicated to me. But some may find this difficult.
It's not only complicated, but it is also
not safe.
Let's say you have this wordlist:
glance merge actual news proof album civilian letter praise fee short responsibility concept stereotype national bad produce flush razor cutting forestry mechanism abuse duty hollow visual spy year plant offender history owl
Let's also say that the attacker doesn't know anything about your system (group of words, random words in between etc), but they immediately understand all those words belong to the BIP39 wordlist.
So the natural thing to do would be to extract all the 12 wordlists from this list and try to see if they form a seed phrase.
How many ways are there to choose 12 wordlists from the list above? The order matters here, so the Permutations are 32!/(32-12)! = 1.08 * 10^17 =~ 56 bits of entropy.
56 bits of entropy is definetely less than the >100 bits of entropy that you can create with a strong passphrase.
If you want to make this method safe-ish you need to create a list of >250 words = 4.55 * 10 ^ 28 =~ 95 bits of entropy. So, creating a piece of paper with 250 words written upon it, seems somewhat difficult, doesn't it? And even then, the "complicated" aspect that many people mention still remains.
In fact, just imagine that if you have all the 2048 words in front of you, then this offers you ~132 bits of security if you wanted to brute force exactly 1 seed phrase of 12 words from scratch. So, finally, writting 32 words, or 50 on a piece of paper, doesn't provide any security.
