Can I apply for the EB-5 visa after being laid off with an H-1B?

Buying a business for $600,000 will not qualify but if you make an $800,000 investment and not out of status for more than 180 days, then you may be eligible for concurrent filing.

Consult an experienced EB-5 immigration attorney since the preexisting business must meet specific requirements to qualify for EB-5. The amount you mention also does not qualify. Under 245k, there is a 180-day exception for AOS based on employment-based filing.

The EB-5 is an employment-based green card program, so if the Priority date is current and you can make the investment, get the I-526 completed and filed with the I-485 you can remain in the U.S. Best though to always have an underlying nonimmigrant status.

The answer to your question depends on where you are from. The bottom line is that if the visa is current, you may concurrently file I-526 and I-485. If you are from a treaty country, you will have another option.

If you are filing for EB-5, there's a 180-day grace period under 245K. It's unclear whether the business you're proposing qualifies though. Neither the investment amount or the structure seems to make sense for EB-5. Maybe safer to make a new investment.