There are many ways to transliterate. While "kh" is probably the most common, I think the proliferation of systems suggests there's no one "correct" transliteration.
The options are "texnika" (eg GOST 7.79 System B), "technika" (eg ISO/R 9:1968 variant 1), "tekhnika" (many different systems, as you mention), and "tehnika" (eg ISO/R 9:1968 main variant, but perhaps the most straightforward in any case).
The options are "texnika" (eg GOST 7.79 System B), "technika" (eg ISO/R 9:1968 variant 1), "tekhnika" (many different systems, as you mention), and "tehnika" (eg ISO/R 9:1968 main variant, but perhaps the most straightforward in any case).