I only want to spend the winters in Bulgaria and keep my flat. Does that change anything for tax?

No. As long as the flat in Germany remains available, residence continues under section 8 of the Fiscal Code, and with it unlimited tax liability on worldwide income under section 1(1) sentence 1 of the Income Tax Act. Half a year spent in Bulgaria changes nothing, because a German residence does not lapse merely because you are elsewhere. That is not a drawback, just a different calculation: anyone wanting the coast as a second home plans without the tax saving but also without the effort. Anyone wanting the saving has to move their centre of life for real. General information, not tax advice.