Just got my first decent credit card after two years of fixing my credit. What should my next step be?

For the past two years I've been repairing my credit after being reckless at the age of 21. At its lowest, my credit score was 520 (now in mid 600's). I currently have five accounts in collections, all due to fall off my credit report in late 2019 and early 2020. Most are paid off. One will remain until 2021. 14 months ago I opened a credit card with Capital One and 10 months ago I opened a Capital One BuyPower card. Both have limits of $500 and a credit limit increase, according to CapOne, is not allowed at this time. I expect them to remain at $500 and do not anticipate any growth in the future. They seemed to have just been credit boosting cards for low credit consumers. I was just approved for a Discover IT card with a starting limit of $1,800 and am beyond excited.

My current stats:
Credit score: 620-660 depending on util rate. At this moment, TU is 640.
AAoA: 5 years
Util rate: 15% - can be 0% by next week
Baddies: 4-5 collections depending on the report being looked at. All are due to fall off in late 2019 and early 2020. One will remain until 2021 but a pay for delete for this one is possible.
Current credit line: $2,800
Inquiries: 2-4 depending on the credit agency report

I want to get one or two more cards before I stop applying and grow my accounts until 2019/2020. I need to buy a new laptop soon and was considering the Barclays Apple Rewards card because it seems like I'd likely be approved for it. The average credit score for approval seems to be 620. I also need a new retainer so was considering a Care Credit card because it seemingly has low approval standards.

I'm not going to ever miss a payment again, and having the Discover card simultaneously excites and scares me. I'm very new to credit. What should I do? I'm a senior in college and earn roughly $30,000 a year.