Welcome to the forum.
Have you checked the fuse at the starter solenoid.
Also check for any burnt connections there.(You mentioned starter solenoid,what did you check? Is there power in and out)
Then check all the earth points on the bike that they are clean and tight.
It sounds like the battery is shot.
There is a low voltage cutoff for your ECU and that will not allow the bike to start with low battery voltage.
There is a probability that the connections to your start solenoid are corroded or dirty and creating a high resistance connection that depletes voltage.
I would remove the battery and have it load tested.
Then, remove the start solenoid and thoroughly clean and inspect the wires. Reinstal.
Check the 40 amp fuse connections and tighten with a little squeeze for some needle nose pliers.
Reinstall a new or freshly charged battery taking note of polarity (negative terminal forward) and ensure all wires get reattached.