Brake Rotor thickness vs size talk

To answer some comments from my original point:

In all cases the brake (always rear) makes a loud screech and I lose almost all power. The lever doesn't feel too spongy, it seems more like the pads can't grip the disc anymore. With the Galfer rotors, you could visibly see the disc warp like a pringle, in fact for 5-10s until I stopped the bike, you could hear the rotor clanging against the caliper each rotation as it was no longer true, you can see heat shimmering off the rotor too.

I understand my technique etc can all be improved, my main confusion is: why did I never observe this from 2008 to ~2022? Especially when one of the main touted benefits of 220mm rotors is better heat management. 

Someone also mentioned improvement in grip from tyres, larger wheels etc. I wonder if before I used to lock the rear wheel more, whereas now, I almost never do. Maybe skids are not just for kids?

Last thought, strava says i'm as fast if not faster now, but I am tragically unfit (gone from 15hrs of riding a week to 2). Perhaps my technique has gotten lazier and I use less front brake which is harder to control and more tyring on the arms, resulting in my now overpowered rear brake getting overwhelmed?

I'll try going back to shimano + 200mm rotors on my next bike and see what happens, I definitely prefer the lever feel they had anyway.

Sorry for the double post, but I happened upon this article with an interesting lab test, looks like my codes and MT5s are among the hottest and worst power brakes on the market... Definitely going to move back to shimano in the future.

https://enduro-mtb.com/en/best-mtb-disc-brake-can-buy/

And think about the difference between front and rear brakes too:

Rear brakes: 

generate more heat, transfer it to the caliper and fluid more  (because they are ‘on” longer). 
Have less cooling from air moving past.
Have less need for high brake force.
If running mullet, smaller rotor (than on a 29er wheel) will generate the same amount of braking force.

Front brakes: 
generate less heat, and do so in briefer hard bursts, so transfer less to caliper and fluid.
Have more cooling from air.
Can use higher braking force.

So, keeping that in mind the optimization can be quite different for front and rear brakes.

Thicker rotors resist heat better. So, a great option for rear wheels, especially on 27.5 rear wheels where you might not want to use a larger rotor to prevent locking up the rear wheel too easily, but you are still dealing with the same amount of heat.

Conversely, in front you might choose a thinner rotor, so it gets up to a high enough temperature quickly, while still using a larger size to ensure plenty of stopping force.
 

Also what pads are you running? We never heard that. All of your symptoms sound like an organic pad to me, and could potentially be solved by moving to a metallic pad.

"In all cases the brake (always rear) makes a loud screech and I lose almost all power. The lever doesn't feel too spongy, it seems more like the pads can't grip the disc anymore."

Your description is exactly what I experience as a heavy guy when I didn't know a bike came with organic pads.

Just too add as a side note. 
I pick up a  year old 10kg 120mm bike with an sl 34 up front from a mate who used to race xc, so it wasn’t set up how I like a bike. 
Bit of  serving and tuning (snapped the front rim in a berm dt  sec1200) race recons f&r for a forkaster and the front wheel from my over built evo with an Agassi. 
So background over, was riding with a shimano 180 mt 905 out back and a 2mm magura mdr 180 up front. Both work great with the grippier tyres on the faster black trails. As soon as I was half way down a proper rooty and rutty trail that the bike wasn’t enough for both breaks gave up (I pull both breaks same time, I know weird) so steep trial most of my weight was on the back so I’d give to the ice rotor.

That’s my two cents.

Louis 👍