I decided after quite a bit of research that Happy Planner could be a really good fit for me. It has a built-in structure to it but is also super flexible and customisable -- all things that really appealed to me. Since it's May and I wasn't interested in getting a 12 month or 18 month planner now (and waiting for next year was not an option!), I decided to build my own. I purchased this notebook as well as a 6 month calendar extension and an extra pack of notepaper, and so far it's working well. I won't use the calendar section until July, but the dot grid sheets have been really useful for daily to-do's, motivational spreads/lists, and therapy notes. I also bought medium discs to accommodate the extra pages.
One thing I think I should mention (and not in a bad way, just a heads up), on my computer screen the discs in this notebook look kind of lime green-yellow. They are yellow.