With a $7,500 bankroll your max bet would be closer to $60 or $75 to have a 5% risk of ruin. A $250 max bet will bring it closer to 75%, which only gives you a 25% chance of not going bankrupt.

If the DD game allows mid-shoe entry then you can make it a little better by Wonging around, otherwise the 6D game will probably give you more opportunities to find good counts.
Also, playing 3 hands can burn through the cards pretty quickly. It’s usually better to play just 2, or even 1 if you are playing by yourself.
-Sonny-