Congrats on your new car and general success so far!
Personally I'd definitely shift into a quiet period for at least the next 6-12 months. I wouldn't apply for anything else or take any HPs (hard pulls) for CLIs (credit limit increases). Personally, I probably wouldn't even do SP (soft pull) requests for CLIs for at least 6-12 months, because I'd rather my profile remain completely static in the near term. This is particularly true if you're already mostly satisfied with your limits anyway.
After about 6-12 months I'd consider doing some SP requests for CLIs and perhaps an HP here or there if you really want a CLI or want a new account. After about a year I'd resume 'normal' credit activity, which for me would mean adding about 1 credit card as desired/needed every 12-24 months. If needed I'd consider doing 1 CLI request per account about once every 6-12 months. I'd stop seeking new accounts or doing CLI requests at least 6-12 months before applying for future car loans or mortgages.
That's just me though and I tend to be fairly conservative with seeking new credit and larger limits.
In your position I'd also focus on paying down your car loan and that 4K in credit card debt.
Also, regarding your gas card if you do want to close the account I'd go ahead. It will remain on your credit report and continue to factor into your AAOA for another 10 years. By then your other accounts will be older and likely more than offset your AAOA. If you don't need/want it and especially if it's costing you anything in AFs or service fees I'd cut it loose. If you still find it useful and it's not costing you anything, then it's probably fine to hang onto it.
Anyway good luck!