This seems to be really difficult. Can see the point of the 2♣ bid, which I intuitively chose, too. Partner will not expect auch an extreme hand when you bid 3♦ as he only knows 6 cards (good point), compared to 10 cards when you manage to bid 1♦,2♣ and 3♦. Thus he will bid 3NT too often when 5 Minor is preferable because he will also bid 3NT with only one stop in the other major, not expecting me too need so much help there (e.g. Axxx Qxx xxx Qxx or Kxxx Jxx Ax Qxxx will not even think about exploring 5 of a minor). On the other hand I think even though 2♣ can be bid with 18HCP partner does not have a rebid with every 8-9 or even 10-count. If he does not have a fit for any of my minors he might pass with say KQxx QJxx xx Jxx or similar, even with a 10-count 2NT would be quite agressive because opposite a minimum 2NT might be in danger from his point of view with 22 combined HCP and no fit... Whereas after 3♦ Partner has an easy 3NT with this hand. In addition to this ops will find the right lead most of the time when I bid diamonds and clubs but will lead a club once in a while against 3NT when I keep quiet about my club suit.... All in all I think it is pretty close. I think I lean towards bidding 3♦ but am not sure if it is really best.