As far as we can see in universe, the tactical benefits of a planetbuster seem to outweight the costs.
There's one benefit to the second Death Star that you're overlooking. The LOLZ. What is the point of running the entire universe if you cant abuse the hell out of your position - and Palpatine is clearly the sort of dude that would enjoy having a whole bunch of slaves and minions build him a space station for the sole purpose of luring in the last Jedi so he can turn him.
Palpatine has the whole galaxy under his thumb. Why not build a big ass vanity project? He's got enough resources at his command to build a dozen and one for his Sunday best and the Empire would hardly blink at the prospect.




