That is the holy grail I have been seeking for the purposes of cleanly editing STL models. I can tell you, that such a piece of software does not exist. The problem is too deep to go into a single post but there was a debate here last year that can be found in this post that espoused a lot of ill-informed opinions on the accuracy of STEP vs STL. That can be found here. The TL;DR version of that post is that STEP models are inherently accurate because they include the parametric data that makes up each shape whereas STL or similar mesh formats such as OBJ are solely triangles linked into a mesh that simulates a solid. For slicer purposes that’s more than enough and all modern slicer programs always convert STEP files into a mesh whereupon the parametric data is lost.
If you want to start out learning CAD, the softest landing in my opinion is OnShape. It is cloud based so it runs inside a web browser from anywhere where Internet access is available. It is also free. Note that I use CAD solely for 3D printing and I have access to Fusion360, FreeCAD and SolidWorks. OnShape was written by former SolidWorks employees and it is a professional-class CAD system designed for enterprise use. The downside is that you must always be online to use it and in the free version, all of your models are shared with the public.
If you want to take a look at 3D Printing-biased review of CAD systems, Michael Laws at Teaching-Tech channel on YouTube arguably is the best teacher(he was a high school teacher) on the subject. He is the High School Science teacher/Shop Teacher I wish I had in High School, he makes things so easy to understand.
Here is his review of various CAD packages and why OnShape one him over. It was the critical video that had me defect from Desktop PC based CAD to OnShape and I have access to multiple Workstation-class desktops so hardware was never an issue with me.
Now here’s where the meat is with your quest to learn and modify an STL file using CAD.
Now that being said, Fusion360 has a STL to STEP tool that in videos looks great but in practice, isn’t worth the $680/yr license in my view for something that still requires hours of tinkering. But for contrast purposes, here is the best video I found on the subject of using STLs in Fusion360. Look at this video and compare it to Teaching Tech’s video and you be the judge. What gets me about all these examples is that the conveniently use a simple model that lends itself to Fusion360’s $680/yr prismatic option but rarely do they show a way to do it with a more complex model.
After viewing the initial Teaching Tech videos, here is a playlist of lessons. Many of the 25 videos on this list won’t necessarily be relevant to you today but it’s a good place to start.