ADHD

I use Google account calendar and tasks. On my phone I use Google Calendar and Tasks.org app, syncing both with Google account. On my computer I use Gnome Evolution for everything including email — in me experience it's far more compatible with Google than Thunderbird.

I have several subcalendars: a personal one for events I add manually, one for holidays, one for birthdays and anniversaries (which is generated automatically basing on contacts data), one subscribed from Facebook events, one for film releases and one for garbage truck schedule. Only the personal one and garbage one are manual, all other are synchronised from someplace else

For tasks I use 3 lists: a personal one, one for university stuff (though I graduated so it's sitting idle) and one for work tasks. The last one is only for stuff important enough to not keep it only on my work phone and Outlook, like sending in timesheet.

Yes I do differentiate between events and tasks because they have entirely different purposes.

Yes it makes sense very much to have a separate holiday calendar because you can easily sync it and not fill in holidays manually and also colour ccode it inside your calendar app