Welcome to the forums,
I think you need to decide which is most important to you, taking care of your retirement or helping your kids with college. Since you listed retirement as number one, I'm going to assume that is.
You say that you keep your finances separate from your husband, but you mention getting an $8,000 tax refund. Is that yours, his, or both? If both, then is only $4,000 of that open to work with on your options?
I calculated your "Available" cash flow after expenses at $2378, $50 less than you did (Salary 4158 minus Mortgage 1230 minus Other 550 = 2378). Maybe small in the great scheme of things, but when wanting to optimize it all matters.
Do not pay extra on your mortgage. The rate is low, and you have other priorities at this point. That extra $300 can be used more efficiently elsewhere. If you are overpaying now, consider making only the minimum to free up more. There are also lots of threads on here about how paying your mortgage down is a mistake mathematically.
Max out your TSP. You can contribute the full $18,500. Your employers match is allowed to go over that. Bump your contribution up to 22%. That will be $1541 a month. If your worried, try running a paycheck calculator to see how things will look:
https://www.paycheckcity.com/calculator/salary/ Throw in different options with TSP contributions and number of allowances.
While we are on the topic of paychecks, use that calculator to figure out how much you should be having withheld. You are correct in that overpaying on taxes monthly is a bad thing. It hampers your cash flow. You can invest it in whatever you want. Plus, it will make increasing your TSP contribution easier.
Next, open a Roth IRA. Get that maxed if you can as well. Depending on how much your husband makes you guys might be close to hitting the income limits for that.
For college, are community colleges an option? In my state students in the top 10%, maybe even lower can get 2 years of community college paid for. If that is a possibility, that can buy 2 more years to save. I would let both of your kids know now, if they don't already, that you might be able to help with college, but will not be able to pay for it all. The younger one should have time to keep their grades up and still possibly get scholarships if they are a great student.
Pay off that credit card debt as quickly as possible, throw all the extra at it if you aren't already. Then get that kitchen loan knocked out. Don't wait until May to do it if you can do it sooner.
Making a plan is a huge step. It's great that you are starting to take control of your finances and plan. It is going to be stressful at first, but once you decide on a path, it will be a relief.