As long as your employer's coverage was deemed "credible" when you 1st signed up and took Part A but delayed Part B (and maybe Part D) there is no problem signing up once your employer coverage ends. It sounds like it should be credible.
Then you have 8 months to enroll in Medicare once you stop working OR
your employer coverage ends (whichever happens first). But you’ll want to plan
ahead and contact Social Security before your employer coverage ends, so you
don’t have a gap in coverage. Just do it so that one stops and the other begins if you can plan such continuing dates - sounds like this will be easy for your circumstances.
! WARNING ! Avoid the GAP and the LIFETIME Premium Penalties:
If you do not enroll in Part B within 8 months of losing your coverage based on current employment, you may have to pay a lifetime late enrollment penalty. (Part B &/or Part D). If that happens, you would only be able to enroll in Part B during the Medicare General Enrollment Period (from January 1 to March 31 each year) and your coverage won’t start until July. This may cause a gap in your coverage and the Late Enrollment Penalty (LEP) would apply.
After that, unless you have retiree coverage, you will have to decide how you want to get your Medicare benefits and coverage.
This Medicare.gov pamphlet should answer your question in detail and tell you how to proceed to become a "full" Medicare beneficiary. WELCOME ABOARD !
Medicare.gov - Sign Up for Part B Package
It's Always Something . . . . Roseanna Roseannadanna