Starting takes a lot of power. Any loose or at least not good tight connections in the battery terminals and grounds will show up particularly when trying to start. Double check that both battery connections are clean on both side of the connectors, and tight. Do the same for grounds.
While you have the battery cables disconnected to clean them, have the battery fully charged and load tested.
With a good battery, reconnect all cleaned terminals and try again. This way you have eliminated the battery and connections from your possible problems, and it is the easiest things to do. Next gets harder and more expensive, so best to start simple and cheap and then move on when those are eliminated!!
Cheers,
TQ