What are my options of re-taking the ASVAB for a higher score as prior service?

The only way you can retake it as prior service is if you enlisted under CAT IV. Otherwise you have to use your already qualifying score and can take the BSEP or FAST class once you get to your duty station.

Talk to a recruiter. They’ll set you up to take one. They’ll use the highest score

Pretty sure you can re-take if you re-enlist