With consonant clusters such as /bst/ and /kst/, we divide them up between syllables so that each syllable is pronounceable on its own (if possible), and preferably consonants go at the beginning of a syllable. We can't say /bst/ or /kst/ at the beginning of a syllable, but we can say /st/, so they're divided 'sub-stan-tial' and 'ec-sta-sy'. In this position, /t/ is unaspirated because it's preceded by /s/ - what you notate as "sd". Compare this to 'obtain', which has to be divided 'ob-tain'. Here /t/ is at the beginning of a stressed syllable so it's aspirated [th].
'Most of' is different. The syllable division could be 'mos-tof', but as the second syllable is unstressed, that doesn't give it any strong aspiration. In fact we can alter the conditions by stressing 'of': put it at the end of a clause. 'We have some green ones, but red ones are the ones we have most of.' And when I do this, and stress 'of', I find I say 'most.of', that is the /t/ doesn't join the next syllable. If anything, it's /mo.stof/, where both /st/ join - and therefore the /t/ is unaspirated, "sd".