This will not be nearly enough. Thessaloniki was on the edge of Slavic settlement, with Slavic apparently being spoken widely there (at least in the 9th century). Far more likely is it's transformation into a Bulgarian city like many of the other cities captured by the Bulgars.
Sources for that? Thessaloniki was never annexed by any Slavic state up that point, it was sieged though and I find hard to believe that many Slavs would be among their midst, especially considering trade was mostly conducted in Constantinople.
The population disparity is not nearly enough. The Slavs likely outnumbered the Greeks on the Balkans.
I'm highly skeptical of that. At least for the first half of the 9th century.
Without them, Bulgarians would inevitably dominate over the remaining Greeks by sheer numbers.
Maybe have the Northern Bulgarian territories be annexed by Magyars or Pechenegs and make them sack Preslav and other settlement in the region, the core are would shift south in Greek territory. Mind that they would also own Athens and Pelopponese.
What do you consider the Balkan Mountains? If it's
this, the answer is that they certainly dominated far to the south of the mountains.
No they definitely didn't the Bulgarian took the bulk of this territory in the 9th century and the Byzantine didn't lose control of it during the Slavic migration in the region, but magically the local Greek disappeared the second the Bulgarians conquered it.
Not workable in an Orthodox country. Perhaps it could remain an archbishopric, which is what happened to the Bulgarian church after the Byzantine conquest.
How is that not workable? For the first decades Greek was liturgical language, it could remain so alongside Bulgarian, especially if the region under its control has multiple linguistic communities.
This includes much of the scientific terminology which is borrowed from ancient Greek. And outside of terms connected with Christianity and subjects which were not familiar to the Slavs earlier borrowings are not that significant
Still borrowings, plus I'm not sure we can take modern languages which have been "purified" during the 19th century, Romanian removed so much slavic influence and both it and Bulgarian(on a much smaller scale though) have so many loanwords from French and other modern European languages that substituted Turkic and influences from other languages.
The Slavs had settled virtually the entire Balkan peninsula, this included most of Thrace as well. And if it didn't become mostly Bulgarian at this point, there would have been little opportunity to become such later.
I mean it had a century and a half of time(and more for some other territories) and then there was the Second Bulgarian empire, which also existed for a century and a half or so.
The thing the territory around the Maritsa and Plovdiv remained under Byzantine control throughout the Slavic migration period, so either Slavs are magical creatures that replace everyone in their way and aren't affected by being under the effective control of such a big empire or probably the slavicization of Bulgarian Thrace didn't happen overnight.