When you say that your Last Touch conversions are low, do you believe that this is because your Marketing Channel Last Touch may be overwritten by another Channel (correctly or incorrectly)? Or do you believe that your Marketing Channels haven't been configured incorrectly?
If you believe there is an issue, either with unintended overwrites, or even intended ones, and you want to see any contribution, you could try applying a custom attribution to your Orders for "Participation" (this will show ALL contributing Marketing Channels leading to conversions).
If you believe that your Display Ads aren't being tracked correctly in your Marketing Channels, this will be bad, as there's no really good way to try and go back into the past to get the information....
It may be a good idea to review your Marketing Channel rules, and also making sure that users aren't using external campaigns internally in your site that could lead to "stealing" attribution....
