Stuck and didn't know what to do at all

So, peeps, I need help. My account level is 25. My main squad already lvl avg 30-ish. f2p player. Done with main story.

I can't even beat Extermination Hard bosses to get Normal Stones to unlock last skills or even Challenge mode to get the Upgrade Stones. Don't even know where to find materials to level up skills. Golds felt so scarce to get, I'm afraid to level up module further especially there's high % chance to fail level up, so all of my module equipped only level 6. Can't even do Special Ops properly because I don't have a lot of characters. I never felt so stuck in any gacha games that I've played before, especially this early. It's extremely frustrating. Now all I could do is fighting chapter 2 boss challenge mode over and over again, plus doing clan rooms. What am I missing here, peeps? Felt like this game is made for whales.

Please comment with tips, info or anything. Thanks in advance.