"ch" is pronounced as /x/ or /ç/ depending on the vowel in front of it, and "s" is pronounced /z/ before a vowel, how come then that the sequence "chs" in the words wachsen and Wechsel is pronounced as "ks"?
Pronouncing "chs" as "ks" is just a simplification that has been made official over the original /çs/ and /xs/ pronounciation, akin to pronouncing -ig as /iç/ instead of /ik/ for the Auslautverhärtung (https://en.wikipedia.org/wiki/Final-obstruent_devoicing#German) or /ik~g/ as fortis and lenis (https://en.wikipedia.org/wiki/Fortis_and_lenis) The proto-germanic "*fuhsaz" still had the /x/: see Wiktionary
Also, many people over here in the south do make the distinction between / ˈvaksn̩/ (to wax) and /ˈvaxsn̩/ (to grow) and also say Dachs, Fuchs, Luchs, with /xs/ and Wechsel, Deichsel and similar words with /çs/. It's just when speaking fast or when you need to be better understandable that one uses the standard simplifications of modern High German in standard federal republic German variety.
You can see the decay of proper German pronounciation the best with words that have a "Ch-" at the beginning like China or Chemie. There's the question though whether either "Schina/Schemie" in the north or "Kina/Kemie" in the south will eventually be used as the new general standard.
"ch" has been developed out of the greek chi. This character has been pronounced in different ways throughout it's development. Even in German, in some words "ch" is pronounced as /k/ (the set of these words depends on the region). So the combination "chs" is pronounced as /ks/. This is a sensible way, as "ch" might be pronounced /k/ and /xs/ or /çs/ are difficult sound combinations for Germans.
"wachsen" is pronounced with /x/ if you mean "(to) grow" and with /ks/ if you mean "(to) wax". Usually, /ks/ is used except when "chs" are part of two different compounds or an comparative/superlative of an otherwise normally pronounced word. In my opinion, using the actual phenom like in "sechs" /ç/ can help avoid disambiguities with identical sounding ones such as "Sex" which is why I tend to pronounce it accordingly except for when I speak quickly.