Or should I somehow concatenate them into a larger singular dataset.

you would benefit from a bigger dataset;

should I go for WordPiece tokenizer for BERT pretraining?

BPE and WordPiece have a lot in common:
https://huggingface.co/transformers/tokenizer_summary.html 36
BERT is trained with WordPiece, so it is natural to choose WordPiece in this case.