The Germans start the battle with a bit over 200 tanks and assault guns (211 to be exact) with the Bulgarians having 60 more. So 10th Panzer should be starting operations reasonably close to full strength. Of course the Allies are now to the southern end of the Monastir gap, there is pretty passable ground without very many obstacles... at least by Balkan standards going all the way up to Monastir and Prilep. And the Allies have 3 armoured divisions that each of them individually start the battle with as many tanks as the entire Axis force in Macedonia. The main concern about an amphibious assault up the Macedonian coast is whether the Allies have sufficient landing craft for one. In addition to OTL Greek yards are churning landing craft at a priority so between that and additional Greek merchantment turned to assault transports by early 1944 there should be about two brigades worth of landing capacity in the Mediterranean in addition to OTL...So there is a breakthrough but not a total collapse yet. Perhaps it is time for an amphibious operation at the rear of the Axis forces defending the way to Thessaloniki! @Lascaris is the German Panzer division in the Macedonian front near full strength or is it a division just in name? If the answer is closer to the latter then perhaps a small amphibious operation will be the solution. I guess the front resembles smt like the crude map below and the operation I'm proposing is noted with the orange arrow.
I'll note that we've seen a German railway officer getting complaints from an SS officer of colonel rank that he's falling behind on train shipments to... Poland. After said SS officer apparently oversaw similar shipments from Constantinople. I would say that the Germans making these shipments from Thessaloniki is anything but good news...The offensive seems to be going quite well and considering the germans are moving their staff back from thessaloniki I think thessaloniki will be attacked soon which is a good thing.
The Allied timetable there is roughly a month ahead of OTL and the Italian co-belligerent army will likely be rather more important given the much larger number of combat troops in the Balkans that escaped the Germans...The Italian front is also going quite better than otl I feel.