You're right that "baths" would not be expected to ever be pronounced with a [t] sound.
It's only possible to have [t] in "months" because of the "n" preceding the "th." When a nasal consonant such as /m/, /n/ or /ŋ/ (the "ng" sound in "sing") precedes a voiceless fricative consonant such as /f/, /s/ or /θ/ (the "th" sound in "thin"), many speakers introduce an epenthetic voiceless plosive. The identity of this plosive depends on which nasal consonant is present: we get [p] after /m/, [t] after /n/, and [k] after /ŋ/.
This means that "month" (most commonly transcribed /mʌnθ/) may be pronounced as [mʌntθ]. Both variants, [mʌnθ] and [mʌntθ], are generally considered to be standard English.
I've never noticed the plural being pronounced as [mʌnts] or [mʌns], but that wouldn't surprise me too much as [mʌntθs] has a rather awkward four-consonant cluster that seems like it would be prone to simplification in a similar way to /siksθs/. This pronunciation is mentioned in another, related question: Do most Americans pronounce 'months' as 'mons', and 'clothes' as 'clos'?
In the word "bath," there is no nasal consonant preceding the /θ/ sound, so you cannot insert an epenthetic [t]. (Speakers of some varieties of English might outright replace the /θ/ in this word with a [t] sound, a phenomenon called "th-stopping", but this is not standard and would not be taught to ESL learners.)
The plural "baths" may be subject to another process however: fricative voicing. In English, words that end in a voiceless fricative sometimes have voiced fricatives in their plural forms. The voiceless fricatives /f/, /s/, /θ/ correspond to the voiced fricatives /v/, /z/, /ð/ respectively.
In cases where the voiceless fricative is voiced, the plural suffix "-s" is also pronounced as a voiced consonant /z/.
For the word "baths," voicing the fricative is optional. So you may hear it pronounced with either /ðz/ or /θs/.
I don't have enough knowledge of phonetics to say if these clusters are ever reduced in fast speech to single consonants [z] or [s]. But I would say your target pronunciation for the word "baths" should end in a cluster of two consonants.