Repairing cracks on deck joists
None of that is concerning. It's called "checking" and it's what happens when wood dries out. Look at the logs next time you're in a cabin. The screws tend to direct it and mildly exacerbate it, but it's not really a problem.

You could use flashing to protect the joists from water going forward, but for every joist you cover there's an unprotected connection or cut end or other vulnerable spot that will rot out eventually anyway. I'd install your new decking and railing and plan on another 10-15 years of service from the deck. Then it'll be time for a rebuild.