What happens to someone with over 18 years who refuses the COVID vaccine/booster? Do they get a mandatory retirement or something worse?

Refusing the vaccine will be a GOMOR if you don't get an exemption. That's policy and non negotiable. Now, if that GOMOR subsequently identifies someone for QMP/QSP, that would be a separate administrative action. People can elect to volunarily retire in lieu of getting kicked out, but you have to be at 19 years or higher to do that (retirement packets for time can be submitted to HRC no earlier than 12 months out, no later than 9).

Good luck with that, seriously.

Veterans benefits for sure but it is up to the command how the discharge is carried out. Retirement I'm not sure about but I would think that would be reserved for those who meet retirement criteria.