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).

In my USAR unit, we had a few COVID vaccine refusers. They are supposed to receive counseling signed by Brigade commander about consequences of not complying. If they want religious or medical exemption, they are supposed to talk to either chaplain or military doctor. If they do not receive vaccine by mandatory requirement date, separation process starts. Actual discharge or retirement type may be dependent on case-by-case circumstances.

I got lucky and two people who had 20+ years service retire in order to save us the paperwork hassle. The higher HQs did us a favor and expedited their retirement packet.

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.