the deadline date for certification from the 18th day after the election (see link). The Trump team had already tried and repeatedly failed with that only legal recourse they had.

Georgia had already investigated and debunked all of the BS claims of fraud that the Trump team put forward. Asking Georgia officials to find more votes after they had concluded investigations of the claims of fraud and certified the results was asking them to do something not covered under Georgia law. The Trump team's chance was to file legitimate claims in court and have a judge issue a court order. Their court cases all failed. Pressuring the officials to find votes after all legal options had been exhausted was pressuring them to illegally overturn an election that had been fully investigated, recounted multiple times, and had objections failed in court.

There is no option in Georgia law for the legislature to send a different slate of electors if they don't like the ones certified by the Secretary of Stated and the Governor. There's nothing in the law that would allow that. Just because you want things like that to be true, doesn't make them reality.