May I ask: why not just keep a first-price auction, and offer proxy-bidding tools as part of the relevant API?
The FPA has some good properties, and is pretty legitimate. A proxy bid optimizer seems pretty self-explanatory… Bidders submit their value to the bidding tool which then computes a best-reply bid given their value.
Also, if people transact sufficiently frequently, you can use tools from online learning to make sure that your proxy-bidding scheme is robustly optimal. Like make sure that bidders have no regret vis-à-vis bidding lower or higher given the realized residual demand curve.