BOK_CHOI

Additional Damage is the most "OP" when you are ATT capped for the raid you are in.
For this reason, +13 lvl 90 will greatly outperform +11 lvl 95 on the majority of the current content.

But since you're also looking into Aes Hithe and Arcana, the oncoming raids...
Their ATT cap I believe is something like 31k?
The further below you are from ATT cap, the less efficient Additional Damage becomes.
So then your priority over additional damage would be hitting that ATT and crit cap (unless you can already hit 31k with your lvl 90, but I'm guessing you're not one of them ).

I've already experienced on my own where a higher enhance lvl 90 would outperform a lower enhance lvl 95 on 23k cap raids, yet the opposite is observed in the Dullahan raid.

So for now I'd say make the +13 lvl 90 weapon first, then work on +11~+12 lvl 95 weapon later, when/if you feel the need (right now it's just too expensive).

My approach was filling in the missing enchant slots on the free armor (even overwriting some of the existing ones according to my need) for temporary use, then slowly working on lvl 95 armor that would eventually replace this free armor.
In terms of weapon, I've just made myself from scratch a fully-orange level 90 scythe (you should be able to make one with just 5~6M now, and it's much easier/cheaper to get one to +11~+12...even though my luck still sucks).

I personally used 5 well-balanced & 4 enthusiastic scrolls for my armor (w/master suffix on chest), along with two fast scrolls on my accessories.
This pretty much solved my attack speed problem, but my DEF suffered a lot...which I've eventually made up for with the lvl 95 armor (good 2k extra DEF).

Oh, and the artifact can be purchased at the Rocheste Castle (the guard NPC).

Hope this was at least a bit of help!

You could also slap on a couple Fast scrolls on your accessories, while also using Enthusiastic scrolls on your armor to counteract the resulting balance loss.
Using multiple Enthusiastic scrolls kills your DEF, though, so you may want to find other means to increase your DEF, such as upgrading to Dullahan armor if you're still wearing lvl 90 armor.