Can you level up to 20 without finishing the story?

Yes. GTA Online is an independent mode. You can begin playing it right away without having to progress through the story at all. There are unique aspects in GTA Online like building your own criminal empire, participating in exclusive events, and interacting with a large community of players that are not related to the story mode progress. So feel free to start GTA Online whenever you like, regardless of your story mode status.

Doing Delve content is quite effective. The deeper you go in the Delve, the more experience and better loot you can get. You can find valuable fossils and resonators which can be used to craft powerful items for your character, while also leveling up.

One of the best ways is to do maps. Maps in Path of Exile offer a great deal of experience. You can start with lower - tier maps and gradually progress to higher - tier ones as you get more powerful. They also drop valuable items which can further enhance your character.

One way is through dungeons. You can queue up for dungeons repeatedly. Each time you complete a dungeon, you'll gain a significant amount of experience. Another option is FATEs (Full Active Time Events). These are events that occur randomly in the game world. Participating in them gives you XP. Also, leveling up your crafting and gathering classes can also contribute to your overall level in a way, as they provide some XP that can be converted to help with combat classes leveling.

You don't necessarily have to complete the main story to start Phantom Liberty. The game allows you to access this additional content at your own convenience. So whether you want to jump into it right away or wait until after the main story, it's up to you.

Doing Gambit matches is a good bet. You get XP for various actions during the match. Also, hunting for lost sectors can be fruitful. They usually don't tie into the story and give XP upon completion.

Well, grinding patrols can be effective. You roam around the map, kill enemies, and complete small tasks which all contribute to your XP gain. You could also join a clan and do clan - based activities like clan engrams. These can provide a significant amount of XP as well. Plus, keep an eye on the weekly challenges that don't require story progress. Completing them can really boost your level.

Sure. Usually, game developers make DLCs somewhat independent of the main story's completion. But it might have some references or benefits if you've finished the story.

Yes, you can. But it might take longer and be more challenging compared to doing side quests and other activities.

Yes, you can. But it might take longer and you might miss out on some rewards and experiences that come with other types of quests.