Is there a way to create a Figure Sheet X of X?

No, it's not built into FrameMaker like the Table Sheet variables are.

But you are aware that each table sheet counter starts over for each table, right? So I'm still not clear why you can't use it for both traditional tables, and those holding the figures. The key is in the variable definition—just leave it at the default of (Sheet <$tblsheetnum> of <$tblsheetcount>) so that the wording will work in both situations. Put your title as described in a FigureTitle (you can use any tag above a table), one figure in each body row, set the rows to start at the top of the page.

Otherwise, I would reconsider the numbering style to take advantage of FrameMaker is meant to handle this: 3-25.1, 3-25.2, 3-25.3 or some variation on that theme. After 30 years in the field, I have learned that it is best if I design around my program's strengths and actively avoid the weaknesses.

If you just need Adobe to change how FrameMaker works, you can always put in a feature request here: Tracker.