CAD

For open source, you have exactly two and a half realistic options: FreeCAD and SolveSpace, or Blender plus a CAD Sketcher addon. The first two use a parametric history paradigm, and need a bit of a rethink coming from TinkerCAD (as I did), but both are more flexible, though SolveSpace doesn't support fillets and chamfers as an independent feature any more than TC does.

FreeCAD is more user friendly than it used to be, but I highly recommend at least one or two intro tutorials for either FOSS option. The time it took before I realized which FreeCAD workbenches to use (and, importantly, which to ignore for my rather basic needs) was rather silly, in retrospect. SolveSpace is very quirky, but I can't recommend its tutorials enough. Even if I didn't quite memorize where everything was, in following them I got a better feel for how parametric modeling actually works.

Blender is blender. If you don't need to move beyond 3D printing, I understand you can get add-ons that make it act enough like a CAD program to make it work for you. I wouldn't LEARN Blender just to use that functionality though. FreeCAD will do it better and support non-mesh models that can be used in other manufacturing methods or be exported to a slicer with arbitrary detail levels.

For Free-as-in-beer, things get more interesting, but pretty much every free tier gets enshittified somehow. Fusion raises prices and hides features behind the paywall. DesignSpark removes interoperability that the free version used to have. Shapr3D's free version is effectively a tech demo unless you are printing literal cubes. OnShape not only lets everyone see all your models, but lets other people (not you) sell them. Still, the odds that someone actually wants YOUR models are low. Solid Edge is just a simple non-commercial use. Either of the last two are at least feature complete for basic part and assembly modeling, as is Fusion (for now)