Dry and sooty plugs (difficult starting)

Initially ran my 02 1150GSA to low fuel levels. Last time parked up facing down on a slope and was very difficult to start afterwards. And since then difficult every time, especially when cold. Thought maybe sediment had got past the filter and blocking the injectors (1200 type installed).
So, replaced fuel strainer and filter in tank. Removed injectors and cleaned, spun engine over and fuel spurted out of open lines OK.
New air filter and plugs put in but, still the same starting problem. Ignition on and I can hear the fuel pump prime.
No CAT but Y-piece and open Z-technik exhaust.
When running and riding, there are no obvious faults showing up and sounds normal. But still running rich.
I get a good spark and fuel getting through to the heads so, what next!
Thinking the O2 sensor is carboned up and I can see soot at the outlet of exhaust.

Could the now dirty O2 sensor be the culprit?
Sending the wrong signal to the ECU.

Once tank is low will remove and detach sensor and wire connection for a clean,

Any aftermarket O2 sensors to consider buying for my bike model from a motor factors?



Patria o Muerte

Sounds like a GS911 would be useful to check fault codes. And maybe a compression tester too.
You mention fuel flowing from the pipes, did you check the spray from the injectors?
It's worth cleaning the O2 sensor.
Also .. grasping at straws .. remove fuse 5 to reset the motronic, clean the throttle bodies in-situ - remove mixture screws, vacuum take-off caps and intake tubes and spray some cleaner there too.