Hm, when comparing Proto-Slavonic reflexes of *tj ([tʲ]) and *dj ([dʲ])
according to this table
seems Srb/Cro stayed more conservative (less language contacts ???) than this change in OCS -> Bulgarian.
Pity our first written document is from late IX century, when OCS already had its recensions.

It is true. Serbo-Croat when seen from the proto-slavic perspective is more conservative in this feature.

Bulgarian/OCS made a first step d+j => dz (đ) like Serbo-Croatian and then made a second step of metathesis dž> žd:

In this respect, the most conservative balkan slavic variant is Čakavian that preserved the proto-slavic palatalized t' (< t+j), which was turned into /ć/ by the other Serbo-Croat variants (cf. čakavski not' for noć).


I see that they have also made the same jazik (post palatal ę>a) variation with the central Macedonian dialects (Ohrid-Shtip height zone) rather than jezik (north) and e(n)zik (south) as in zajak (when compared to Zaječar and "Egejčki" zaek)