Just see your Career Planner for a Medical Extension if your EAS gets close and you're not getting anything back from the PEB. If you get ratings and accept MMSR-4 will adjust your EAS to within 60-90 days.
They used to involuntarily med hold Marines, but know they make you do an extension for LIMDU or PEB anticipated past EAS.
Based on what you said, I think you'll have to do this extension.
You should be using the NAVMC 321M form for Medical Extension if the Career Planner isn't aware of the change.
The NAVMC 321M form is available at:
https://navalforms.documentservices.dla.mil/formsDir/_NAVMC_321M_15675.pdf