For Unity I changed my play style from other AC games. In the past, I would try to clear out an area- do all the side missions, get the collectibles, etc- before moving on to the next. But with Unity that's really hard so I just did kind of whatever I wanted at the time, focusing on the easier activities first so I had time to level up.
Here's my recommendation:
Filter the map. You can choose which icons are displayed, so just choose the stuff that interests you. I recommend Paris Stories, Murder Mysteries, and Enigmas, because those are actually fun things to do, not mindless collectibles. Once you take out the collectibles the map starts to clear up. If you want to pick up collectibles while you're running about doing other stuff of course that is up to you (that's basically what I did).
Now once you're in sequence 3 or 4 or whenever you finally get Arno's phantom blade, start tackling some of the easier side missions and activities. Difficulty is marked by the dots, 1-5. This way you'll earn some creed points and money and actually start enjoying the side content (hopefully- if you're not enjoying this than probably the game is just not for you).
Don't worry about the currencies, just play, and you'll acquire money and points for equipment upgrades. And don't worry too much about the details of the equipment upgrades either- I just chose ones with increasingly higher stats that looked nice to me. This isn't like Skyrim or whatever where it really matters so much which equipment you use.
As for not caring about the story line, I can't help you there, it's a pretty important part of all the AC games. I don't mean the modern-day arch story line I mean what happens to the character you're actually playing, like why you're running around killing or saving this or that person.
Take your time. This game's strengths is savoring the atmosphere and situations and setting, not rushing through objectives on a map.