What kind of UCMJ exists to protect civilians from the enlisted (ex. if an E-5 assaults a civilian, how would they be punished)?

UCMJ covers all Active Military Personnel. It covers only military personal. If a Soldier assaults a civilian on post the Soldier could be charged under the UCMJ. Off post, the Soldier would be charged by the civilian authorities. Sometimes, those off post cases are passed back to the military for adjudication. Much of this is based on local and state law, and the Assimilated Crimes Act.
https://www.justice.gov/usam/criminal-resource-manual-667-assimilative-crimes-act-18-usc-13

If you know someone that was assaulted by a member of the Armed Forces she simply needs to file a complaint with the local police. Full stop. The police will investigate and coordinate with the command and CID as required. The DA and the CJA will decide jurisdiction. More than likely the local DA will take it. The complainant gets no vote on the process. If there is no evidence for probable cause, neither is likely to take it. If a soldier is convicted in a civilian court, the Army is likely to chapter the soldier. Depending on the file, a 13c. If there is no prosecution, the command may elect administrative or non-judicial action. Any of this is murky from the question in the thread.

If the E-5 in question assaulted a civilian off of a military installation, that E-5 can face charges of assault in the civilian court and then face UCMJ action when the civilian authorities turn the case over to the military authorities. Now, the possible charges the E-5 can face on the military side are:

Article 128: Assault

Article 92: Failure to obey order or regulation (possible)

Article 116: Riot or breach of peace (possible)

Article 124: Maiming (possible, depending on the severity of the injuries)

Article 134: General Article (the catch all if no other Article fits)