You can't send mail to your own characters, so you'll be forced to know someone you can trust on both servers to get that money from your main to your alts and back.

Materia is probably your best bet. And patience. It'll probably take you 1-2 weeks to buy up all the materia you need (of a type that moves enough to make it worth while) and then another 1-2 weeks to sell it all at prices that mean you don't take too much of a hit.

But, in the end, expect a hit. I'd imagine that out of the 8 Mil, you'll be lucky to still have 5-6 Mil left after this process.