With all common conjugation technologies it is simply not possible to achieve 1:1 coupling. If you wish to invest heavily in antibody engineering, you could insert a single reactive group e.g., cys or azide, in a defined position, or insert a motif that allows enzymatic incorporation of an oligo label.

In general, standard methodologies are perfectly fine. If you really want one oligo per Ab, you could still avoid a lengthy engineering project and attempt to purify singly modified molecules post conjugation, though not all molecules would have the same point of attachment.

Nick Gee Thanks for your suggestion! Next, I will attempt to collect the different components through chromatography.