frawjōn

No comments about zacht, but vijf and Süd are somewhat irregular.

vijf

foif#Alemannic German isn't really "North Sea Germanic", I suppose. Earlier Appendix:Proto-Germanic/fimf mentions "an irregular consonant change" from

Appendix:Proto-Indo-European/pénkʷe, which gives

• Appendix:Proto-Celtic/kʷinkʷe, which gives
  • Gaulish: pempe
  • Brythonic:
    • Breton: pemp
    • Welsh: pump
• opposed to
  • Goidelic: *kwe(n)kwe
• and Italic, which gives
  • Oscan: pompe
  • Umbrian: pumpe
• opposed to
  • Latin: quinque#Latin

I think Germanic:fimf, Celtic:pempe/pemp/pump, Italic:pompe/pumpe differ in a visibly obvious way from other Indo-European languages.

Süd

I would almost dare to say that sunt#Old High German is the odd man out. We have suðr#Old Norse (a bit early for Low German influence) and Romance languanges often have sud/sur (even sud#Romanian)