Cfrgtkky

In my document I restart the chapter counter for every part. Unfortunately, this leads to ambiguous numbers for, e. g. figures. Therefore, I decided to add the number of the part as a prefix to the chapter with:

renewcommand{thechapter}{thepart.arabic{chapter}}

In general, this works nice. However, I have some chapters before a part which leads to a prepended ".", e. g. ".1 Chapter 0".
So, the first thing I want to achieve is removing the point when there is no part above the chapter.

The second issue arises with the appendix because the appendix chapters are not included in the "renewcomand" above. It would be more uniform if the Appendix, which is part III would be numbered like "III.A", "III.B" and so on.

Here is the current output:

How can I remove the leading "." for chapters outside a part? How can I add the part number to the appendix chapters? Ideally, this should also work for the figures (see "List of Figures" in image), tables and equations.

UPDATE:
Concering the first issue ("."). I found this solution:

makeatletter

renewcommand{thechapter}{%

    ifnumc@part<1 @arabicc@chapter

    else thepart.@arabicc@chapter

    fi

}

makeatother