‘The Cambridge Guide to English Usage’ refers to words that ‘relate ideas to each other, helping to show the logic behind the information offered’ as conjuncts. Now, as used in the OP’s example, seems as if it might fit this description. The LSGSWE (‘Longman Student Grammar of Spoken and Written English’), however, goes rather further, and places such words under the overall heading of ‘Linking adverbials’:
The main function of linking adverbials is to clarify the connection
between two units of discourse. Because they explicitly signal the
link between passages of text, they are important devices for
The LSGSWE identifies six major semantic categories of linking adverbial: enumeration and addition, summation, apposition, result/inference, contrast/concession and transition. Now is given under the category of transitional adverbials, but it seems as if the authors have in mind such use as ‘Now, that may be true, but . . .’ It’s more likely that the use of now in the OP’s example is as one of those linking adverbials that 'mark a concessive relationship: they show that the subsequent discourse expresses something contrary to the expectations raised by the precedingclause.' Now is not given as an example under that category, but it seems similar to one or two that are. In particular, it seems to perform much the same function as terminal though.
Now as used in the OP’s example is likely to occur only in speech, and, as such, its sense will be enhanced by prosody rather than by punctuation.
(For anyone unfamiliar with it, the LSGSWE is the stripped-down version of the magisterial ‘Longman Grammar of Spoken and Written English’.)