What Lascaris said. Northern Thrace is pretty solidly Slavophone outside the major towns, but just like rural Macedonia, 'Bulgarian', just as 'Greek', was an unknown concept to most of the rural people until someone came (a priest, a schoolteacher, a gendarme, a university-educated youth who propagated 'modern' concepts like nationalism) who told them that they were, in fact, Bulgarian or Greek (or Serb, or Romanian). There are plenty of sources attesting to the fact that many of the Balkan peasants conceived themselves simply as 'locals' or 'Christians'. Of course the Bulgarians have an advantage that far north, just by the absence of many Greek-speakers and distance; but TTL Greece has a thirty-year head start, so their influence will be felt much further north than OTL.
They will definitely want it, but it looks like Greece will get it first, long before the Bulgarians have had a chance to develop their position there or get a border close enough to claim a share; then that will be a fait accompli, just like the 1885 Eastern Rumelian annexation (which is the analogy I was driving at). IOTL the contest was about who would control southern Macedonia and Salonica, but even then, the Greek propaganda network was active in areas further afield, in what is now North Macedonia. The way things are going ITTL, the contested zone will lie (much?) further to the north than Thessaloniki. Plus, ultimately, schools and propaganda are one thing, but where the border ends up being will be determined on the battlefield, just as in OTL, where Greece arguably was much more successful than expected.