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Also the approach you describe of free plugins for proprietary services may be better served by paying for Plesk.
It is likely that you are never going to find a single WebGUI manager that will let you do everything from one interface. New services and solutions will appear, or you may prefer one particular provider that doesn't provide a plugin. In general proprietary services don't make plugins for FOSS software early on. If they appear it would often be a community effort that takes time to mature.
The real answer is that you should know what is going on underneath your WebGUI. Learn how the systems work from the commandline and you will have a better time long-term.
All of that being said there are these other OSS management WebGUIs to look at that may give you more of what you seek:
You will have to find out for yourself how lively these projects seem and whether they are investing time into.