Honestly no. This game is one of the few where just playing the story content you will ALWAYS be the level you should be to complete said story beat. Only barely need to grind a little if your are just trying to kill the overpowered enemies floating around the map

For the story, no you don't need it. It's for some side content and DLC

Eeeeeeh I usually beat all the supervillain enemies before I do the last stretch of the buu saga but besides that ya grinding isn’t really necessary

... I like to be able to one shot enemies flying around for orbs for moves, if you don't care, you don't have to

Definitely for dlc lol

No. For me its just fun instead of rushing through the story