Your examples are the only 2 ways to add dynamic text.
Tool Tip ... sometimes I create a scatter graph (based on MIN(Number of Records) on both shelves) and choose the lightbulb shape, then write the "help text" in the tooltip. You can then reference values dynamically and customise to the user. Pro: It doesn't take up a lot of space on the dashboard, and the user can ignore it if they know what the text is. Con: You have to know the text is there and hover over the shape to read the text (in a smaller, moving window).
Text on the dashboard ... usually a graph cut down so that you can only see the title (usually) and sometimes see the text content . Pro: "in your face" so you don't miss it at all. Con: takes up (valuable?) space in the dashboard area.
What's best ? It depends on what you're trying to achieve and what's best for your users.
Dynamic commentary is always an interesting subject; depending on design, some of the ideas you suggested are certainly tools that can be handy. I tend to rely much more on worksheet text tools for commentary - this means Annotations, Tooltips, Titles and Captions; using an independent worksheet as just a text paragraph can work out too since there is rich text editor to call on multiple fields as well. Worksheets tend to be tied to a data connection and as such can easily reference data from that connection in a dynamic manner. Simple dashboard text boxes or titles will not give you the same ability to reference fields dynamically. It ultimately goes back to design choices - I prefer annotations as it forces me to be concise and specific about the data.
Another area to look into is the growing Natural Language Generation space; its a bit more advanced but being able to process visuals back into commentary is a growing area that has worked well with Tableau extensibility story. You may take a look at this Wordsmith+ Tableau demo here to see how other tools can work with Tableau's JS API to provide more written description: Bringing the power of natural language to Tableau | Tableau Software
thank you. noodling...
?can I put a tooltip on a title?
?can I have multiple tooltips on a worksheet?
You can't put a tooltip on a title, but you can use a worksheet as a title (which of course can have tooltips associated) - there's a lot of these little tricks and design choices that we've seen people take advantage of in order to provide the functionality desired. You might take a look at this and see if it can apply: https://www.interworks.com/blog/anonymous/2012/01/13/instruction-tableau-dashboard-hidden-tooltip. I've seen a lot of dashboards enhanced with provided a small icon fro instructions or just commentary that might not need to take up additional dashboard space.
As for multiple tooltips in a worksheet - tooltips are generated by each axis on the view (since a new marks card is generated with each axis measure). If the view is has two axis or leverages Dual Axes; each mark generated for each axis can have its own distinct tooltip still.
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