bessarabia

According to this scenario, as a result of the 1812 Treaty of Bucharest, the Russian Empire annexes the Moldavian Principality in its entirety, and not just its Eastern half (which later became known as Bessarabia) as it did in our timeline...

what is the best time or PoD for Russia to annex Wallachia and Moldova/Bessarabia (and possibly Transylvania), be it by conquest or political marriage? Would such an annexation be of interest to them? Could they trade their possession of Finland for such territories or could they annex them...