Hecatee
Donor
Thing is that you then begin to have a problem of distance. If you look at pre-colonial eras you see a tendency for policies to reach a certain size which rarely go above a given point which is linked to speed of information. At one point information takes too much time to come to the decision centers and the state can't react properly so great losses are endured. A Tyras - Vistula border is one of those distances because the Vistula does not lead to a point where easy communications back toward Rome do exist in the way the Elbe or the Oder do come back to the Danube at place where roads going south are available.Oh please, you Plebians have yet to get on my Romaboo level.
The fact you don't even consider a Tyras/Vistula river border shows how un-roman you are.
Let's look at Orbis data for travelling time in the Ancient World (orbis.stanford.edu) :
We see clearly that the area around the Elbe or even the Oder seems to be in the same range as Britannia, but the area around the Vistule is clearly much more difficult to reach. So I'd consider it a worse border. A good question though would be wether or not a move of the borders to the Vistula might cause a change of the capital to Athens or Constantinople. If the west was completely pacified up to at least the Elbe and most of the legions were on the Vistule and in the East then we could easily see a necessity to move the court (I dismiss Sirmium due to no sea access, but it could also be a city on the Danube... Aquincum for instance, especially if the pannonian plain begins to produce a lot of grain : the issue with the capital is the amount of food needed and how to carry it there, thus why a sea city is better to get access to both the african and egyptian grain)