The “age of grace” ends at the rapture.
I understand where “that which restrains is no more” is interpreted. It is a long stretch to say there will be believers in the trib and no Holy Spirit indwelling those believers. We need a bit more scriptures than that one reference to come to such a bold conclusion.
A sure fire way to determine if this is the case is to expound on the Gospel the two witnesses will preach as well as the 144,000 sealed of Israel. If we say they bring a different gospel than the NT then we put ourselves in the error of the Mormons and others.
I tend to not let eschatology drive Soteriology.