I do not extend my desktop to another monitor, but it would seem to me, you should first and foremost disconnect the monitor and adjust your settings so that everything appears as it should using only the native resolution of your laptop. There are many things that affect the appearance of fonts and icons. Resolution, Scale, text size, advanced display settings, themes, the system font, independent setting within apps (especially browsers), hard coded apps whose appearance can not be changed at all, and graphics card settings.
Once the laptop alone displays correctly, then you can troubleshoot any problems when you connect to the monitor. Once the monitor is in the picture, it's own settings come into play. As an example, my monitor has it's own settings where I can make even more adjustments to the appearance outside of Windows.