I have the nightclub. You can fast-track relationships on the dance floor. First, get a sim to become the DJ. Then have other sims click on the dance floor. However, if your sims are enemys and they dance together, they will become disliked because the status bar only goes up. i.e. good friends to best friends.
I didn't know that there was a career associated with the nightclub so I can't be much help there. If you are really stuck on a goal for days, I recommend you just use 5lp and skip it. You will gain it back through more goals. I have skipped tons of goals and bought most of the themed houses and still have 300lp.
The only way to be a bartender that I know of is to have an adult sim active and click on the bar area. The app is kind of picky where you click though. Once you find the right spot, an action will come up saying something like "tend the bar for 20 minutes". Once you click that, the sim will walk over and spin to change clothes. After that, activate another adult sim and click on the bar surface. Again, it is a bit tricky to find the right spot. An action will come up saying "order cocktail $2500 for 30 seconds". After that, the bartender will shake a glass and give your sim a wine glass. At the end of 30 seconds, your sim will drink the from the cup and be inspired.
Other actions: sit at the booth (max 2 sims at each booth + must be adults, not preeteens or toddlers)
I hope this helped!