Can I transfer Roth assets to my brokerage account, without transferring pre-tax assets?

No. Transfers to the self-directed brokerage account work in much the same way as transfers to any other fund available within the plan; any transfer will include a proportionate amount from each contribution source.

For example: If 60% of your account is Roth and 40% is pre-tax, and you transfer $1,000 to your brokerage account, the transfer amount will include $600 of Roth assets and $400 of pre-tax assets.