Personally, both as a consumer and as founder, I feel like intros like these come across as a bit aggressive, like I can almost read into the email that you want to sell me something. I prefer an intro of the lines of "Hey, glad to see you on board, I'm the founder of $company, let me know if you have any questions or if you just wanna chat, I'm around". Not that I have the data to know which works better, but to me it makes more sense that the intro is about just talking about the product rather than how can we get you to buy from us best.

I feel like I'd probably parse that message as non-information, since I know I can send them an email if I want to (but I probably won't).

What has led me to submit feedback most often is having a fairly prominent "send feedback" button somewhere on every page. It means as soon as I notice a problem or think of a useful feature, I'm immediately able to press a button and write even just a one sentence suggestion.

I would never respond to that type of email. I would however respond to the question posed by the grandfather.

Much better is "Hi, I'm the founder of Acme. Can I buy you lunch and talk about your thoughts on our product?"

hm... I like this. :) Does it work? Have you tried?

There aint no such thing as a free lunch.

What email subject line do you find works best in conjunction with this email?

Anything short and casual like "Question" or "Reaching out." If the subject is short enough then the body of the email (the actual question) will often be visible in the inbox preview, letting people see it's just a short question and not a long and robotic message.