Can I reverse the removal of a stolen iPhone from my Apple Account?

No. There is nothing you can do. If you had a passcode set in the phone, it's unlikely that any of your data was exposed. What you have done is removed the activation lock so that the thief can use or sell it..

Is there anything I can do now to reverse that?

Sorry, but no.

Did removing it expose me to further harm?

No, the data on your iPhone would still be protected by the passcode if you had not already erased the data on the phone, but whoever has the phone can now start over and erase the phone and set it up again as their own phone if they wish, or sell the phone to another person without worrying about Activation Lock.

Are there additional steps I can take to protect myself?

I assume that you contacted your carrier when the phone was first missing, to ask them to "blacklist" the phone so that it would be of virtually no use to anyone who had the phone. If someone tried to set up the phone again, they would not be able to obtain cellular service from any carrier who honors the blacklist.