I think having a chat interface is part of it so it’s friendly and convenient to use. I think subverting suggested a matrix bot and that would continue the theme of privacy. You could have different channels or groups for different personas. Maybe one is your Majordomo, another is a simpler calendar bot, another is for journaling, etc.

Then, wire in some cron jobs to invoke the AI to prompt you on various subjects. “Anything you want to share?” every day at 9pm. If you miss a day or two because you were busy or asleep, you can catch it up later or not at all. You could even have it ask what you did at work if your life is like mine and they kinda blend together and you don’t know how many hours you worked.