Sounds like you have all the usual suspects already corrected.
The O-rings on the bottom of the slide guides will not normally be responsible for a lot of fuel leakage. But they do affect the carbs performance a bit.
The guides were designed as the "soft-part" in the carb that takes all the wear from the slides vibrating in and out on intake cycles from the big cylinder. They can have significant wear and cause problems with the fueling if badly worn.
To remove the slide, the needle jet (long brass tube) must be removed first.
They can be a bit stubborn so what I do it this:
Unscrew the main jet about 1 1/2 to 2 turns.
Tap the main jet toward the top of the carb with a small soft hammer or even the plastic end of a screwdriver.
This should start the guide moving upward toward the diaphragm area.
If more "persuasion" is necessary you can GENTLY thread in a 4mm screw (I think this is the correct size) that has longer threads and give it a few more taps. Careful here because I don't believe the M4 screw has exactly the same threads as the main jet and you do not want to "rethread" the brass needle jet.
If your slides require replacement, I was able to get mine from a BMW dealer. They are the same ones used on the early model BMW Funduro 650. BMW has these listed as a part number (slide and O-ring" while Suzuki makes no mention of this.
The last thing I'd recommend is to find the carb vent tube that exits the between the two carbs and is usually routed back under the seat somewhere near the airbox.
If you ride off-road or dusty conditions, a small air filter (K&N has them) clamped onto this line will prevent dirt from entering the area under the diaphragms of the carbs. Maybe you found dirt there while cleaning?
Also, On reassembly, particularly in cold weather, I'd recommend removing the bolts holding the airbox/rear fender to the frame. This will allow you to move these bits upward and back a few cm. This will give you more space to refit the carbs and make CERTAIN the boot between the carbs and the airbox is sealed correctly on the carb mouths.
Doing this seems like it takes longer but, in fact, will save time and bad language while wrestling the carbs back into place, pinching and tearing the airbox boot etc.
Lastly, make sure the carb vent tube and vacuum line (the one driving the fuel pump) are connected to the carbs BEFORE you install the carbs on the bike. There is very little space available for installing these lines after the carbs are in place.