Need to consider three things:
A) business model (what actions do you consider valuable to you)?
Is this a social site? Is it a lead gen site? Are you trying to convert people to FD? Are you trying to retain FDers? Who you are largely dictates what you'll use to watch.
B) what do you need to look at?
Are you concerned with engineering metrics? revenue metrics? AARRR? How site speed affects perfomance? There's a whole range of things that GA and homegrown tools can cover. What actually matters to your business?
C) for whom is this data, and what are they going to do with it?
Is this data for investors? For the exec team? For the marketing team? The engineering group? What action do you expect to happen when people consume whatever data you're going to be crunching?
Happy to talk more about it, but it may need a phone call or two.