What to do after finishing the story campaign?

If you're really only interested in stories, then the game ends with Resurrection for you.

As you've stated, this is mainly a multiplayer game. Galactic Assault and Starfighter Assault maps have some "backstory lore" behind them (e.g. the Death Star II map is about a crashed Rebel ship's survivors trying to flee from the station), but these are just there to provide some context and atmosphere - it's nothing compared to what you're looking for.

I recommend you to play some ARCADE modes.

It is very simple but fun. 🙂

And you can play more contents (like Villains/Ships etc...) what are not included in the campaingn.

Hello @EmeriousNine  happy to hear you enjoyed the story! I liked the set up of the campaign too and how you progress through some of the maps but didn't like the turn in the story that much myself.

However, as was already mentioned here, there is no more campaign content then what you have now, and none will be added to the game in the future (as was said so by Devs long time ago). There are some nice arcade modes though that you could try out and see if you like the multiplayer mode GA, in which every map has its own story around the objectives. Although you said always walking the same path seems boring etc, I ensure you that GA has some great maps to enjoy. Objective based game modes in SWBFII are built around the immersion of star wars so I would definitely give, at least GA, a try.

Later this year (somewhere fall probably) a new single-player Star Wars game will release, titled Jedi: Fallen order. Not much details are available yet but I am looking forward to another SW story!

Play campaign again on a higher difficulty.  Im with you though i love the stories, but ive accidentally fallen in love with completing milestones in multiplayer mode.  Give it a try.