From an economical point of view, chosing this editor seems pretty reasonable. He is probably experienced with the field and thus probably best suited for selecting referees or may even decide to review the paper himself (and then probably hand the paper over to another editor). In both cases and on average, this speeds up the review process and raises the quality of the reviews.
However, the editor may be biased regarding the importance of your work, which however is no real issue if the work is clearly over the journal’s relevance threshold. Moreover, the editor may overprioritise it and be more lenient towards it. All of this is in your favour and I would argue that dealing with this is the editor’s or some supervising editor’s job and not yours. One could however also argue that you should select another editor for this reason.
Just selecting another editor may however also have negative repercussions, as editor that raised your conjecture may feel omitted or you may be regarded as having been sloppy when choosing the editor and thus wasting the editors’ time.
Thus I would suggest to elaborate either decision of the editor in one brief sentence in the letter to the editor (or the journal’s equivalent), for example:
Note that we did not choose X as a handling editor because we already suggested him as a referee.
Note that we did not choose X as a handling editor to avoid conflicts of interest.
Should our choice of X as a handling editor be regarded to cause conflicts of interest, we kindly ask the journal to choose another editor.