That is definitely the next thing I would try :) mostly the reason why I started with a BiLSTM is that it's much easier to implement/debug, also afaik the time

That is definitely the next thing I would try :) mostly the reason why I started with a BiLSTM is that it's much easier to implement/debug, also afaik the time complexity of RNNs with respect to the sequence length is O(N) but it's O(N^2) for attentional models like a Transformer. Although it probably doesn't matter much on the scale of the SST-2 dataset.