You don't get to determine what is an "authentic Premil." As far as I'm concerned, the vast number of the early Church Fathers were Chiliasts and Premils. The fact Lacantius came when he did is all the more convincing of this fact since he indicated his view had been dominant.
Immediately after Lacantius relates these things he says this: This is the doctrine of the holy prophets which we Christians follow; this is our wisdom.
Sounds like he is pretty convinced that Premil covers the preceding era as well as his own?
Totally disagree. Where is your evidence? I will not hold my breath. You make these claims yet fail to carry through with hard evidence. Your claims are simply based on discredited "historians" like Thomas Ice. That says it all! The pattern is: one partial Premil advocate bases his views upon another partial Premil advocate in order to build a false narrative. The reality is: Amil was the predominant viewpoint between AD30-AD430.
Lacantius is no different to any other ancient or modern commentator, he believed his view was the biblical view. He didn't argue anywhere that his position was that of the ECFs. You forced that on his comments. You continually foist your narrative upon historical data. You have to.
The fact is, you have finally found your first Premil over 200 years after the cross (Lacantius). What you negate to tell the reader is that he built his eschatology upon the heathen Sibyl prophetesses. This confirms the Op and shows the dubious origins of ancient Premil.