Fixing up a rusty old Rallye mountain bike

I bought a Rallye mountain bike from Facebook Marketplace last year and did most of the major overhauling then as well. It was covered in mud, gunk, rust, had broken pedals, etc. So I had replaced the grips, pedals, brake cables and scoured as much rust off of it as I could.

Now that I’ve finally had time to work on it again I’ve found it’s rusted again (it’s been sitting outside exposed to the elements, I know it shouldn’t be kept out there). The brake handles are cracked worse than they were when I got it so I ordered a new pair since it prevents me from actually using my brakes they’re due to arrive in a couple days, and I haven’t been able to shift since I’ve gotten it so I’m trying to figure out how to get the derailleurs working again, there’s significant slack in the cables.

In all honesty, I’m a bit of a newb pertaining to bikes but it seems simple enough since I’ve gotten a handle on the bare basics. Any and all help or listed resources will be awesome.