Just finished the level 99 grind on an echoing strike build, and I am huge fan of insight for a cost effective weapon that can clear P8 in a reasonable time. The quality of life from meditation aura is great, the implied critical strike is also significant for outright damage. I swapped to an eth thunder maul fortitude for the last few levels and did find it better overall - more resistances, more damage (most P8 things are getting 1 or 2 shot at that point), while still having FCR to hit the 125 breakpoint for fast teleports and attacks.
That said, you can get an insight going in an eth shillelagh or elder staff or CV for next to nothing, while a eth thunder maul fort was trading from 2K-5K depending on the days over the past week. So, YMMV based on your wealth on whether you want to splurge for slightly better or put your gold/item wealth towards other upgrades. Building a Pride for your merc is a significantly better damage upgrade than going from Insight to Fortitude.
I know youtubers put out damage calculations on eth arioc's being the best, but we don't play this game in a spreadsheet. The QoL you get from easy FCR on weapon is massive, and more useful than overkill damage. I did Uber Ancients and Uber Tristram with the Insight build and had no issues.