I vacillate between BusyCal and Fantastical (amongst others) on both iOS and macOS; I find I can add a short or even long set of notes in the manner you describe reasonably well (especially when combined with Keyboard Maestro macros and voice-to-text); with the added advantage that all the data is not only synced instantly and flawlessly using iCloud, but is absolutely universal and compatible with any CalDAV calendar, including the stock macOS/iOS Calendar, so you can switch at will and not lose data. But, sadly, on its face, in most standard views (Month, Week, Year) neither displays said notes without first highlighting “an event”; but it is available in Day view in some apps (sadly, not stock macOS or iOS Calendar).
I pretty much have the exact same need to record symptoms/conditions on a running basis, as well; so I have resorted to creating a calendar called ‘Symptoms’ (and others for similar use); then I just quick-add “events” that begin with the text I require to describe that an event or notable condition occurred, and if needed, add additional text to the Notes field. Again, using macros, you can easily keystroke or speak-command your way to rapidly fill the desired entry fields.
Then, I can easily toggle the view off/on said calendar(s), so as not to interfere with viewing traditional events and appointments; on iOS, I use one app to display and enter my “normal” calendars, and another strictly for journaling symptoms/conditions/other, only because toggling calendars to be viewed is generally more tedious on iOS. I could do the same dual-app approach on macOS, but I use keyboard shortcuts to quickly toggle certain calendars in a single app.
I have experimented with bonus features in third party apps, such as Sticky Notes in BusyCal, and, they are interesting, and might offer you another approach to adding larger amounts of text directly to a given date, or even to an event/time.
Finally, we get to the display aspects; in BusyCal, e.g., in addtion to seeing notes inline using Day view, you can View as List any given calendar(s), and choose which fields are displayed, and in what order, and at what width. If you just want to review onscreen, this is generally very good, except that long notes do not text-wrap, and appear on a single line, and are truncated if you run out of space. However, if you print (either to paper or PDF/Preview.app), the truncated fields do indeed text-wrap, so you are able to create a really pretty decent journal of events (symptoms, conditions, etc…), including date, time, duration, location, etc., etc., for consumption or presentation to others.
Oh, and BTW, you can, obviously, share these specific calendars, for either view-only, or give others the ability to add/edit as well. Very helpful when multiple caregivers are journaling for one or more patients.
Once assembled, I find being able to present a list to my doctor(s) listing all pertinent data (especially time and duration, and relationship to prior or subsequent events), has been absolutely invaluable to tracking, isolating, treating, and even curing issues; as opposed the the vagaries and vagueness of “Oh, I think I felt that pain five or six times last week, but at different times, and no, I can’t say for sure what I ate that day or if I just finished exercising, blah blah blah.”
Now, all that said, I’m still looking for ultimately better solutions, too; I really, really need a comprehensive, health-focussed journaling app for both macOS and iOS that allows me to quickly enter BP, weight, oximetry, symptoms, meals, calories, time medication was taken, water consumed, current mood, etc., etc., and then log and present it just as a journal would (in addition to isolating, charting and graphing trends) and most importantly these days, also sync data to Apple Health.app, which, as it matures, is going to make getting this data to your doctor(s) much easier to coordinate and monitor your care. Right now, macOS is (as usual the past decade) getting short shrift, and doesn’t even have a Health app viewer, let alone an app you can use to enter data. Heck, it doesn’t even make any sense that Health.app isn’t even available for iPad, where you could at least manually add data, and more easily view it all, too. It matters not one whit that iPad doesn’t have the same sensors, and you can’t pair with Apple Watch (which is also fairly arbitrary).
My current world is a combination of disparate apps to enter vitals and calories and symptoms, et al, and every once in awhile, for some data, I have to go to each app (or sometimes just Apple Health.app on iPhone) and add some of that more easily recorded AND charted data (e.g., BP) to the appropriate calendar in Calendar (BusyCal, Fantastical, Calendars5, Timepage, etc…) on iOS, or macOS, if convenient, as most of those specialty vitals/calorie apps (e.g., LoseIt!, MyFitnessPal) do not easily export data in a useful form for automated import to other apps. I have at least eliminated all but one app that doesn’t integrate directly with Apple’s Health.app.
Some other hacks I have used are using Siri to quickly create a new Reminder, which helpfully also appears in Calendar, BusyCal, etc., and can be added to a specific list/color, so I can then transfer those into the “journal” of events in the proper calendar later.
Other fantastic hacks on iOS come in the form of using Drafts (4 or 5) to create super-fast calendar/journal entries and have them automatically added to Calendar (et al); and Workflow (soon to be Shortcuts in iOS 12) along with Siri should also make this as all as easy as saying, “Hey Siri, add, ‘I’m having chest pains right now’ to Symptoms.” ; and through a combination of automation magic, do exactly what you expect; i.e., change ‘right now’ to the current time, and create an entry in your ‘Symptoms’ calendar. It should also be possible (as it is on macOS now) to have the language parse a time, title, as well as extended text into the notes field; add duration, location, etc.
Sorry this is so wordy; I just want you to know you’re not alone; and pretty much exactly what you want can be done right now, even for free using AppleScript and Automator on macOS; and for minimal investment on iOS; a decent amount of investment on both platforms can greatly minimize the pain you’re currently experiencing. If you’re interested in shortcutting, automating, keystroking it all to make it easier, we can continue in more detail.