Nuclear umbrella with Pakistan or China sounds good on paper, but do not think it is practical. Bangladesh having its own nuclear weapon is even more impractical, unless we want to completely ruin our economy with possible sanctions. US will never allow another Muslim majority country to obtain nukes.
Best deterrence for Bangladesh is its population. India would never want to capture and absorb 170+ million potential hostile population. Nonetheless, should there be any possible misadventure courtesy of radical Hindu leader in future, I believe a strong conventional defence is deterrent enough to deter India.
Bangladesh needs to be more wary of Indian covert anti-Bangladesh activities and strengthen State mechanisms to counter them. We ought to focus on cleansing DGFI and DG NSI of BAL and Pro-India loyalists first. And then strengthen them to counter Indian covert activities in Bangladesh.
We don't need nukes.