There’s a lot to unwrap right here. I will go over the factors that want addressing, most essential first:

Do not attempt to invent your personal cryptographic scheme

When you learn the guide to your pockets, it most definitely says to generate a random mnemonic, and explains why you may (and may not) wish to use a passphrase. The producer/developer is aware of very effectively why they let you know this, and until you might be an knowledgeable (and doubtless even if you’re), deviating from these directions means your safety will vary from suboptimal to nonexistent.

Safety degree

3,54 × 10¹⁹ works out to be just below 2⁶⁵, which implies you are solely getting 65 bits of safety at greatest. A 12-word seed phrase has 128 bits of safety, which is similar because the safety degree of the Bitcoin protocol itself. In case your scheme does not obtain at the very least that, your safety degree is degraded, and 65 bits is low sufficient to be breakable in apply with sufficient computing energy.

Brainwallets

65 bits of safety assumes that the 8-character passphrase was generated randomly, i.e. one thing like by7rkf0a, 2xp5u7wf or 63tf3sv5. However that is not what most individuals want to use as a passphrase. In a extra practical state of affairs they’d use e.g. a reasonably frequent phrase (assume prime 10,000 English phrases) and three random digits, decreasing the safety degree to simply 47 bits, which may very well be brute pressured in an inexpensive period of time on an everyday PC.

A pockets generated from a human enter is known as a brainwallet, and far has been stated and written on why brainwallets are unhealthy. If you wish to study extra, this 2013 BitcoinTalk put up by Greg Maxwell is a wonderful start line.

Checksum

Small element: the final phrase of a 12-word seed phrase incorporates checksum info. It simply so occurs that “true child” repeated six occasions is a sound seed phrase, however this solely works with round 1 out of 16 pairs of phrases.