Some things aren’t clear to me from reading your initial post.

what is your end goal? Why do you want to score the likelihood of sentences?
why are you set on using a MLM-model like BERT/RoBERTa and training it yourself for an autoregressive problem? Why not use pretrained GPT-2 or the like for this?

The CoLA dataset (corpus on linguistic acceptability) tests for grammaticality/acceptability, which may be what you are after