Taking over the whole Moldova with what is there is going to be an even taller order than effectively harassing Ukraine's logistics with Romania... Especially with Ukrainian forces ready to flank them.
The whole Transnistria has something like 1 standing brigade equivalent, 3-4 additional of crappy infantry if they mobilize reserves. While Moldova has little more in terms of standing forces, 3x the reserves, they aren't going to get surprised now, so they can't get blitzed, not by forces like that, and if Ukraine intervenes, Russia is taking an L because the whole place is cut off from anything new getting there from Russia.