OP - You have plenty of time. If you are flying UA, when you are through CBP at IAD, head upstairs and ask UA if they will stick you on their 5:30 PM to CLE. Don't ask for SDC as it's not SDC-eligible. Just ask if it's doable and see if it can be done without a fee. If it can be, it's your lucky day.

After a TATL flight, with another flight to go, I would not fancy Washington public transportation which is not exactly designed with the long distance traveler in mind.

Uber is easy and relatively cheap. OP has more than plenty of time. If there's traffic, and there will be, just sit back and relax.