You can absolutely refer to constraints from other sketches in expressions, copy geometry directly from other sketches and "export" geometry from other bodies or generated geometry with a shapebinder in Part Design (or a Facebinder from the Draft workbench in a Part workflow). Shapebinders are very useful.

There are master sketch workflows. Youtube videos are out there for those (Mango Jelly Solutions has at least one).

The “losing faces” issue is the Topological Naming Problem. This is very soon to be mitigated and it can be worked around in the meantime (offset your sketches from a base plane rather than attaching them to a face). Though it is I gather still murderous in one of the assembly workbenches. The fix is coming soon.

Many of your other concerns would be addressed by doubling down on the parametric stuff, I think. The Spreadsheet module can help solve a lot of problems that are otherwise solved by the methods above and may be easier to keep track of. You can do calculations in there more or less as you would in a typical spreadsheet, and then give cells of the spreadsheets aliases that you can refer to in expressions anywhere in your design.

FreeCAD is really fundamentally parametric, and once you get into that, you have a bunch of options.