My TU FICO went up over 150 points from when I got my IT Secured (around 550) to now (701) and no CLI love. Discover is definitely NOT just FICO based for credit limits -- I know at least 3 folks with secured->graduated IT cards who now have higher limits than I do with way lower FICO8 TU scores according to Discover's free pull monthly.
Discover is super weird, I had to fight tooth and nail to get a $2500 limit with a 780 EQ 8 when I've had my $15000 BoA CR for 5 years. Computer only approved me for $2000, and analyst sounded genuinely excited he got that increase to $2500 lol...
One of the theories I've seen thrown around is they want to be your daily driver and don't want to have to compete for usage, but that doesn't seem to hold up in your case either. It's also definitely not income based given what I've read on here and my own experience. Yeah Discover is a weird animal. :/ Right now I'm just using it for the 5% categories, and maybe will switch back to it once I've hit $5000 spend on the Uber card so I get 2% general cash back.
All Zero Except One.
Basically you only let one card report a balance to the credit agencies every month.
Best to keep it around $10-$20. you want the balance reporting to be under 9% of the cards total limit