Their are ecosystem companies\partners\VARs that do this.
If you really wanted to customize the data you could feed your AWDB\HDS to a shadow or copy database then query against that copy DB to create your own reports and dashboards if you dont want to use CUIC. Along those same lines a strict Read Only account and firm control of who can run what types of queries against what and when can technically be done. It is important to note that everything in this specific last paragraph could have horrible and environment breaking impacts on your call center let alone not being supported by CIsco or TAC and is not in anyway something I am suggesting.
The point being what CUIC is doing is querying a SQL DB and wrapping, along with a few other things obviously, it in a UI so any of the normal ways you can create reports against a SQL DB are also an option.
what are the ways to customize reports & is there a tool that simplifies customizing reports
Not really. The "tool" is CUIC itself.
Customizing reports is really changing the "View" - removing fields, adding missing fields, renaming, resizing, formatting, adding thresholds and so on. Not that hard; but it takes a bit of fiddling and trial and error.
But what normally happens is you can't get to where you want to go because the SQL query does not do what you want or expose the fields that you need.
Now you are on a totally different train. Now you need your T-SQL skills, a deep understanding of the schema, and a fairly decent insight into the day to day running of a contact center.
If you can create a T-SQL query to show the data you need, you can certainly create a Report Definition from that and abstract the inputs through the Value Lists making it have a wider application. It's hard to show you though - you simply have to try and fail, and try and fail and get up again and try once more.
If you don't know your SQL and don't want to learn, then buy services from someone else. It's a very steep hill to climb. It's not rocket science, but it may seem that way to a novice.