So a rough scenario for an eventually independent Dalmatia has a PoD in the early 9th century. Istria is returned to the ERE under the Peace of Aachen in 812 tripling the population of the Theme as well as doubling its economic power and size. This allows the Archon in Iadera to exert influence over the Dodge as his inferior rather than an equal as the position is seen as a successor to the former exharhate. The communes along the not adriatic communes rather than striking on their own become perferal elements of "Dalmatia". The third and forth decades of the 9th century are not marked by conflict between Venice and adriatic Slavs but between "Dalmatian" archon and all his resources against the Slavs, culminating the defeat of Duke Trpimir of Croatia in a land battle ensuring Croatian leaders would look to expand their power elsewhere rather than towards to coast. That does not prevent slavic settlers from the hinterlands to move towards the coast and become inhabitants of Dalmatia. Using the string of successes the Archon is able to ensure that Dalmatia is elevated to the position of a Theme in the 850's, a generation earlier than OTL and is able to capitulaise on other events that happen it the period such a slow collapse of Imperial authority in the Franksih Empire and the Bulgarian raids against Croatia that ensure there is no Croatian ascension as was the case in OTL. By the end of the century using the renewed interest of the ERE on their western holdings Dalmatia secures all of the islands on the Adriatic and expands the territories in the hinterlands of the coastal communes as well as acquireing a few communes in the south of Italy following successful wars against the Saracens.
The first 1/4 of the 10th century is a period of stagnation and great danger as the first Bulgarian Empire subjugates the interior and threatens the communes. Split, Ragusa and Iader are besieged several times, but the navy ensures the sieges are pointless. This has two effects, the Dalmatians decide that they sea power and trade will be the pillar of their power while aslo acquireing territories inland to serve as a buffer zone. After the collapse of the Bulgarians the Balkan interior sees a number temporary warlords try to establish their hegemonies that usually fall apart upon their death or soon after so we get period where there is Rascia ascendant, then followed by Doclea, then Zahumlye, then Croatia, the Doclea again, the Bulgarians etc... it is a period of great instability.
Preying on the weakness of the their neighbours the Dalmatians slowly start absorbing bordering shires (zhupanije) creating a buffer zone between the coast and the main masif of the Dinaric Alps. By the time Turks overrun Anadolia and create the largest crisis in the ERE in centuries Dalmatia is the principle trading power on the Mediteranean with a sizable population, great wealth, decent land holdings and a navy second only to the Imperial one. As the Imperial authority weakness the political structure of Dalmatia slowly moves towards a republican form drawing inspiration roman history, after all the people living in Dalmatia apart from calling themsleves Dalmatian primarily use the term Latins when representing their identity. The political elite starts to see themselves as the true cultural and political successor of the old Roman Empire since it is the only major part that remain Latin speaking as opposed to Greek.
With the 12th century fast approaching and the Crusades on the horizon the future is bright for Dalmatia.