You can NEVER 'sketch...randomly' in ARCHICAD. You will run into what you have experienced. It will be more work to ever get wall lengths, angles, etc correct than to just re-model properly.
You must use the Tracker (or the older Coordinate Palette) as you place or adjust elements. Enter the exact number that you want, and it will be that.
You can NEVER just look at the Tracker, as you look at temporary dimensions in Revit, and "see" a value and click and expect it to actually be that value - unless you've snapped to a hotspot on another element.
I've never used grid snap, and find it an interference to working. Some types of construction, it will make sense of course. Accidentally turning grid snap on is a common beginner mistake resulting in elements that are not the desired size. Modeling precisely - and then snapping to hotspots of existing elements - will guarantee your success.
One of the forum moderators
AC 28 USA and earlier • macOS Sequoia 15.2, MacBook Pro M2 Max 12CPU/30GPU cores, 32GB