If you want Darius to be good in high elo, buffing his damage is the worst possible choice

Buffing Darius's damage does nothing to make him less exploitable in high ELO and more valuable in pro play. Darius already does plenty of damage innately and his season 8 shift to Trinity Force over Cleaver has only reinforced that.

If you want Darius to be less of a shit pick the higher up in rank you go, then buff his his E.

For instance:

• In season 7 there was an undocumented range nerf to his E by about 15-20 units. The indicator never changed so the actual range of it is still shorter than the hitbox tells you it is, and it was an all-around unnecessary nerf when Riot had already taken out his Q mechanic involving 4 stacks of passive as well as nerfing the inner damage of Q from 50% to 35%. Revert this and you can ignore the point below.

• E has a 24 second cooldown. For being his primary fighting tool in lane, this is simply too high, and instead of buffing this, you keep raising his damage. This does little to solve his glaring flaws in higher play (only fight-forcing tool on such a high CD compared to other lane bullies who are either more mobile or outrange him and all of which have far less conditions to their damage to begin with, e.g Renekton, Riven, Kled, etc.), but it makes him extremely abusive in low ELOs who don't understand these nuances and this reflects honestly, probably all the way from Bronze to mid-Diamond. Players below the highest ranks will play a lot sloppier and give him more opportunities to trade and take free shots than he really should have, and their lack of coordination makes his 'scaling' into late game much, much better than he has in higher ELO unless his team is built around him. They don't play around his very long E cooldown, which is his only way to guarantee his combo of W+Q, so he looks a lot stronger than he should be.

As a Darius player myself, I strongly advise against giving him more damage. This will make him far more hated in lower ranks of play and across the general soloQ population but will very unlikely make him look appealing in coordinated play, because just stat-sticking Darius with more number buffs isn't going to make him look like a good choice.