How to fix bad extrusion on my printer
It appears that at a certain point, you're running into issues with the extrusion being a bit too viscous / melted causing it to over-deposit or string (fail to retract completely). It also looks like this issue gradually gets worse as you go. This really feels like nozzle over-heating to me.



My troubleshooting steps would go like this:


Make sure my cooling it set correctly and fan is working properly. The nozzle doesn't adjust its temp for changes in extrusion speed. At times the printer has to rely on the cooling to get the extrusion leaving the nozzle to an ideal deposition temp, which is far lower than the temperate of the nozzle.
Check my tool paths to make sure I'm not starting to return to the same X/Y position too immediately when I reach certain layers, not allowing the previous layer to fully solidify. Consider tweaking the slicer or printing at a lower temperate.
Check if there's a lot of variance in print speeds for different tasks (shell, infill, etc). It can be hard for some printers to maintain the right temp for the extrusion about to be deposited if the speed changes too rapidly. If your printer has been heating extrusion rapidly for quick movements and then the speed slows way down, it can struggle to keep the subsequent slow filament from getting overcooked, as it can't instantly cool the nozzle even with proper cooling. Bring the various extrusion speeds closer together.