Left Mouse Button not working consistently.

The basics on how a mouse button works in most is is that the plastic button of the upper part of the mouse presses down on a microswitch that sits on a PCB inside the mouse.

There can be two initial possible causes for this:

1. A fault in the microswitch that is under the left button. They are rated for a lot of clicks, but eventually they will wear out.
2. Another possible cause is that the plastics of the mouse may be broken inside and it may not be hitting the microswitch properly anymore.

A good way to check is to open the mouse and inspect the left and right sides if there are any visual differences, or if you see any broken plastic parts or wear, or if directly pressing the microswitches feels different.

While there are no guides specific to this mouse, there a scroll wheel guide, but it will at least show you how to open the mouse and take a look inside: Logitech G305 Scroll Wheel Replacement

The buttons are in the upper part of the mouse casing, the bottom part of the mouse contains the main PCB and scrollwheel.

Take care when opening and don't force anything, mind the cables that connect the two parts.

You can typically open the mouse up enough and still have it active so you can test the switch directly without the plastics.

If you think it is the microswitch, it will depend on how this mouse is built how easy it is to replace this part. From an quick inspection online, you can get whole replacements on sites like AliExpress that just screw in, so no soldering may be needed.