Reddit mentions of The Ever-Blooming Flower Garden: A Blueprint for Continuous Color

There is a really handy book called "The Ever-Blooming Flower Garden" that shows a systematic way to ensure that a flower garden is continually blooming. It's a really useful and approachable garden design method, and I refer to it a lot. I don't like paying full price for books, but I'm glad I made the purchase. The specific cultivars listed may not be readily available, but the book explains the plants in enough detail that someone relatively familiar with ornamental plants can make substitutions.

As far as a spreadsheet for flowers - there are probably millions of flowering plant cultivars, so you're not going to find a bloom calendar that has that much data on it. If you need suggestions for something specific, though, people here might be able to help you.

If you're also vegetable gardening, I love Smart Gardener. I use it for garden layout. I don't think it's customizable enough when it comes to scheduling and reminders, but they're making big improvements right now, so hopefully it'll have improved functionality in the future. I think it's the most intuitive to use and shows the most promise.

For scheduling, it's fiddly, but I just put things into my google calendar. Here's what it looks like. Here's the detail of one of the "sow indoors" reminders.

Planning out successional vegetables is really helpful. I used to plant a few carrots thinking I'd do more the next week, and then I'd forget for a month. I have ADHD, though, so reminders are more important for me than they are for other people.