IMHO the big picture is missing. Examples of Indic scripts are found inside of China, as far northeast as central Vietnam, at some (later) sites in Japan and Korea, and all over Southeast Asia. Most of the use of these scripts is related to religion and to a lesser extent record-keeping. Unfortunately, the majority of these scripts were written on palm leaves which do not keep very well, so we are only left with the relatively rare inscriptions on stone and inscriptions on metal.
The Indian seafaring polities which are associated with Southeast Asia (ie. Srivijaya et al) are from India's south and eastern coasts. The south of India is also the traditional home of the Tamil, a Dravidian (non-Aryan) people whose language and script predate and differ from the prakrit languages which dominate Aryan-influenced northern and western India. What is interesting about the Tamil is that they had a record keeping culture in which inscriptions were made in to metal plates which were then bound on giant keyrings like a giant keychain. There are a great deal of these preserved. One wonders if zero exists in this corpus, whether or not it goes back to a directly contemporary period. If not, then the origin would appear to be clearly non-Tamil and could be ascribed to the subset of enthusiastically seafaring empires of the southern and eastern Indian coasts. The apparent main Tamil/Dravidian seafaring polity was called the Chola. https://en.wikipedia.org/wiki/Chola_dynasty but there was also the Pandyas https://en.wikipedia.org/wiki/Pandya_dynasty and Cheras (west coast, modern Kerala) https://en.wikipedia.org/wiki/Chera_dynasty
The Indian seafaring polities which are associated with Southeast Asia (ie. Srivijaya et al) are from India's south and eastern coasts. The south of India is also the traditional home of the Tamil, a Dravidian (non-Aryan) people whose language and script predate and differ from the prakrit languages which dominate Aryan-influenced northern and western India. What is interesting about the Tamil is that they had a record keeping culture in which inscriptions were made in to metal plates which were then bound on giant keyrings like a giant keychain. There are a great deal of these preserved. One wonders if zero exists in this corpus, whether or not it goes back to a directly contemporary period. If not, then the origin would appear to be clearly non-Tamil and could be ascribed to the subset of enthusiastically seafaring empires of the southern and eastern Indian coasts. The apparent main Tamil/Dravidian seafaring polity was called the Chola. https://en.wikipedia.org/wiki/Chola_dynasty but there was also the Pandyas https://en.wikipedia.org/wiki/Pandya_dynasty and Cheras (west coast, modern Kerala) https://en.wikipedia.org/wiki/Chera_dynasty
Some of the non-Tamil seafaring polities: the Pallava, the Gupta (central-northern India), the Pala (Bengal coast and northward). https://en.wikipedia.org/wiki/Pallava_dynasty https://en.wikipedia.org/wiki/Gupta_Empire https://en.wikipedia.org/wiki/Pala_Empire
Any of these could have been the Indic culture at which "zero" moved eastward with merchants or religious record keeping.