Your last sentence/question is your answer Nat. Little things make ISTJs happy. Sending cards, doing chores, and helping out in little ways takes some of the work off the overworked ISTJs shoulder makes ISTJs happy. Remember, ISTJs have the "Guardian" SJ temperament. This means they are a little more traditional and conservative. Doing traditional male/female roles is what she is accustomed to. So, I would answer and ask questions to her in simple yes/ no answers and be clear and direct and concrete. I know this because my boss accountant was an ISTJ. Practical uncomplicated jokes help lighten her up as well.

My Point: Just do little things for her. Holding doors, being a good conversationalist, doing chores for her without telling her or doing them with her is a good way of saying you are trustworthy, loyal, and dependable.

Opposites do attract though. I don't think she would be a good match for you. Plus, being traditional as she is, she might not like the idea of a guy and woman just living together. It may be "inappropriate" to her.

That is all for now. Good luck.