smaak
{{inh}} was fine, it was *smakkuz. Modern long vowels come from either Proto-Germanic long vowels/diphthongs, or from short vowels in an open syllable. In Middle Dutch, these two types of long vowels are still different (we write the former as â ê ô and the latter as ā ē ō) and some dialects such as Limburgish still keep them apart. From a Proto-Germanic short vowel + geminate consonant you'd expect a short vowel in modern Dutch.
Since the modern word has a long vowel, it can't come from *smakkuz but must come from a form that satisfies either of the long-vowel criteria. Sadly, Limburgish has lost the word, and has borrowed smaak from Dutch, so it's no help here.
As for schmecken, it's from Proto-Germanic *smakjaną, a class 1 weak denominative verb.
@Isomorphyc, FWIW, the *smakkuz entry suggests this became Old Dutch smak, matching the closed-syllable pattern of a short vowel, and this then turned into Middle Dutch smake. That -e on the end suggests that the term might be parsed as sma + ke phonologically, making the a now an open syllable.
@Rua, am I parsing that correctly? And is the Descendants list correct as shown in the *smakkuz entry?