My preference would be to start the new fork asap. It is not just up to me though and takes a lot of coordination among the witnesses and stakeholders to pull off.

In terms of preventing witnesses from ignoring transactions in the blocks they produce, there is no way to control that at the code (i.e “hardfork”) level. Witnesses will always have the ability to choose what transactions to include in their blocks, as long as all the transactions they include follow the consensus rules.