My left click started having issues where it doesn't register unless I press down hard like the OP mentioned on my 2 year old M65 Pro RGB. I actually soldered new switches with CHERRY Mouse Micro Switch DG2 T85, but the problem is still there. So I am thinking the problem is not with the switches. It's weird because it works perfectly fine when it's working and the issue is very random, like out of nowhere it stops registering the click unless I push down harder. I also had issues with the right click where when I am holding it down during a game like to zoom in weapon scope, it'll keep zooming in and out of the weapon on any FPS game I play. However, this issue was solved by replacing the switches.
Edit: My click issue started after 1 year of use which I ended up buying G502 Hero SE. My son needed a mouse so I had to give him the G502 which is why I tried to fix the M65 Pro RGB. Now I am using my old 2012 Razer Mamba that I had for almost 9 years and it's still working solid without any issues. I will probably buy another corsair mouse since my entire PC parts are mostly Corsair and uses ICUE. Does anyone have any recommendation? I am not going to get another M65 as this issue seems to be very common.
Edit2: I was able to fix the left click issue by reseating the top shell and screwing down the two screws really tight. Everything seems to be working good now!