I am not so sure byzantophiles here should cheer for the downfall of the HRE... Triunes are in much better position to move into Germany in case of collapse and Germany and Rome realistically don't have that much conflict of interest (disregarding Theodor's crazy ambitions).
Triunes are clashing with Romans in the east, not HRE. And Balkans have enough space to clearly delineate Roman and HRE influence.
I'm not saying that there isn't a benefit to having a German Empire that is in that position geopolitically - but it suits Roman Interests for them to be interested in the GERMAN side of Europe, rather than the Roman side of Europe, and the Carpathian-Vienna-Alpine border really is the best way to ensure that Roman interests in Europe are best fortified. After all ITTL, the Latins have been more than a bit of a sod. Making it so that the Romans are effectively unassailiable in Europe, and have no other interests IN Europe basically sets a new status quo for a long-term workable peace. The end of the HRE is mostly titular in nature and is the icing on the cake. As you said, Germany has little reason to have a conflict of interest once that border is in place. Having the border in the Balkans makes for more potential conflicts over the Balkans.
Have a winner, have Germany focus on N.Europe, Romans on S.Europe, and work together to limit W.Europe. It wouldn't be the first time two great enemies became allies.