New player, level 52, need some advice.

I think getting Phoenix to 7* is a good primary focus, as you cannot do a heroic rancor raid without having 1 7* character. Heroic rancor will give you Han Solo shards and is another one of those useful toons that can plug into a variety of arena teams. Having 5 at 7* with good gear will also allow you to get a 7* emperor and thrawn, as well as complete galactic war, which can help provide credits and shards. I agree with the people who mention scoundrels as a side project. You still have quite a few levels to go before you will be required to have 5 for the highest level of credit heist and training droid heist, so they can be farmed slower.

As for guilds, it is good to try to find one that is close to full and has active members. You may want to inquire about the average raid tickets the guild gets per day. The maximum any guild can produce is 30,000 raid tickets a day, which would be a heroic rancor raid ever 2 days, so if you want the most gear and shards, you will want a guild that averages as close to the maximum as possible. Most also suggest trying to find a guild running the highest raid you can qualify for, as the rewards are better at the higher levels.