Yasmin - As I'm sure you know yourself, the problem here is one of organisation which FH has many ways of managing, depending on personal preference and circumstance. However, there may be no substitute for some methodical filtering and research which the tools in FH can help with. At the moment you can't see wood for trees, so you need to take a deep breath and chop down a few branches one at a time!!
Without knowing the detail and trying to keep it simple, I would not split the tree as everyone else agrees. I would go through all the individuals and mark those lacking information/needing research with an appropriate flag and add them to a list as you go. So maybe you might have a list for those who need their identity checking, another for those who simply need more detailed research or individuals relevant to a records office. Just don't end up with too many lists and start off with bite size chunks!
The new Notes function in FH7 is excellent, allowing you to add research/to do hashtags to notes and the records window has a separate tab for Research Notes which I find very useful when those ideas strike and I know I will forget them all too quickly!
I wouldn't delete everyone who isn't directly linked to you. Some of my most interesting lines of inquiry have been with distant relatives and surprisingly often events link back to closer connections or illuminate family stories.
I would also echo using charts to get a feel for relationships and using the hide branch buttons as necessary.