Putin threw the “Novorossiya “(Donbas and Lugansk) experiment under the bus when Crimea happened and a short time later casualties started to become excessive- switching to Syria to change the narrative. The idea of Novorossiya was dropped and Russia did the bare minimum, just to be a spoiler, for 8 years. I think they were reluctant to add these territories fully into Russia(annexation) because then they would be responsible for all the pensions etc there. Keeping them as quasi states is more convenient. And Ukraine was a lot, lot weaker 8 years ago, if he cared he would have done something then. They are just means to an end. The Kerch bridge would not be easy to take out and they may want it as an escape corridor for civilians out of Crimea. But at some point- yeah it could be targeted.