I don’t know if you actually realize how serious your case is. You keep asserting your marriage was real and so didn’t need a lawyer etc however your opinion doesn’t matter, what matters is USCIS’s opinion and clearly they’re not convinced which is why your approval languished. I don’t buy your explanation that your I485 was pending so long because of a joint sponsor.
This is how USCIS will view your case: After the implosion of your first marriage (which wasn’t approved), you expeditiously lined up a second American citizen spouse and filed another petition. Does it happen in life? Absolutely however it’s suspicious and you can bet the scrutiny will be focused on the first marriage since apparently the second has solid bonafides.
You can continue the DIY route however be aware your case is neither standard nor will the sailing be smooth.
I wish you well.