But when you go from S1 to C1, think differently, what practices would you, as a normal person, employ? I said it's simple and straightforward, but put in a little bit of interference like moving positions as well.
I can think of nearly infinite "simple" ways, that's the problem.
Correct, it's 5 letters for each seed word, not 6 letters, you're one step closer.
Your wording is a little ambiguous. Do you mean that it's five the letters that reveal information about the seed word and one letter that's irrelevant to the word? For example, the first four characters of the word, and one character for its position ("aren1"). This is what I understand so far. First five are related with the word, and the last one perhaps has to do with the silver card placement.
It's not actually relying on a third party, it's being indifferent to a third party, that's not the same thing, as if I had to lock every seed phrase card into a safe and now I can put it in a desk drawer and just put a piece of paper in the safe that explains the rules.
The difference is that the drawer is yours, whereas if Google decides to ban your account, or even shut down their Drive service, you can no longer access the data.