well the thing is with reavers is that they can kill a zealot, DT, or ascendant in 2 seconds (4 if you heal them after the first shot), where a colossus only does 120 in 4 seconds, not enough to kill. Of course the minor downside is damage is more spread out among your units, but that's what you have an AoE heal for. I'd rather fight 2 colossi (even if they had thermal lance giving them 9 range) than a single reaver.
I am curious as to why there's no colossi in this mission, they'd be pretty good against your army without being 1-shotty like reavers. Could also run multiple of them in a single squad for DT's to go to work.
I don't feel the reavers are too terrible to deal with, buut the 9 range (compared to your max range of attacks abilities of 6 and sight of 10) means that you're going to get surprised a ton). Attack speed is the wrong nerf, and is relatively meaningless imo. I'd much rather have a flat range nerf and re-positioning across all difficulties so a) that there's some time to react b) you can see them before they start their attack animation (looking at you, left base reaver).
The 2 main ways to deal with reavers:
Alarak charge->push->flee. I showcase this method at 4:00 to destroy the cannon. Note that alarak only takes 1 hit. Downside is that reavers will move a long way before heading back, can be a bit of a surprise.
Blood hunter disable. 2 blood hunters can permanently disable a target, and you can have 4 when you hit the first reaver
The boss melts to blood hunters.
Overall, a bit too reliant on reavers to slow the pace of the mission down.