Player Scheduling Tool for West Marchesque Campaign?

I have roughly a twenty players in our extended group to pull from, and any time 4-6 are available, I'd like to run a single-session adventure in my campaign. We use Discord for general communicating between sessions, but juggling all the available/not-available messages is tedious (especially when they are subject to change), and Google Calendar isn't cutting it.

Anyone know of a good tool, with low overhead to let each player mark available dates and times a few weeks in advance, so we can lock in our sessions? Ideally:

• WordPress plugin or Discord bot (our campaign tools are in a self-hosted site). Not a deal breaker though.
• Very easy to use. While I am, not all our players are technological adept or used to scheduling/meeting tech.
• Allows dates and times
• Allows each player to change their availability
• Allows each player to add a few notes/comments