At the same time, the FBI needs to watch their arse. They are doing scary things to get people to spill the beans - especially about trump. But that only works if they find something- for example on Manafort- that they can make stick without his cooperation and then try to get his testimony as part of a plea deal. All though it may not be true of all of them, many of the people who Mueller will try to shake down are cooperating because they actually believe they did not do anything wrong and want to clear themselves. If hassle them too hard, then their attorney just says "fuck em" plead the fifth, stay quiet, let em go to trial with it if they think they have something, and helping them to find stuff on trump is not your problem. Thats the problem you run into if you approach everyone with a gangbusters approach. All your potential cooperating witnesses just lawyer up and plead the fifth and tell you to F off.
Although it is a bit marchiavellian, it is actually good for trump if they have stuff on flynn and manaforte- two people who trump distanced himself from after about a month with them or some short period of time. Trumps lawyers mission is to protect trump. He knows there is goiing to be some cannon fodder because meuller is not leaving town without at least three or four indictable bodies. Otherwise he ends out looking like an impotent fool. He has a quota he has to meet even though his hoity toity professional arse would bristle at that term. Flynn and Manafort are givens. Muelller will dirty Trumps kids and Jared but nothing will stick. And mueller will find a couple more victims by looking at old business deals. Then he will ding trump on a whole raft of shiite for being unethical, questionable, blah, blah, blah but wont be able to make it stick.
Note that Mueller's original mission was a counter-intelligence investigation. As far as investigating the dnc for leakers there, or uncovering deep staters assisting leaking and security breaches, meuller will do absolutely none of that. You will learn about trumps business deals in kazarkifuckistan fifteen years ago but wont learn a single thing more about alleged russian hacking. Yet the whole purpose of the investigation is to leave the country more protected against russian hacking which has not been proven and the investigation will not even look at. Oh that's right, Clownstrike- which is on the dnc tit- looked at it- but the FBI was denied access.