I can't send the link to YouTube ?

I have m861, can´t say its very stiff, they changed only rear triangle of m868, so likely the same. Depends  what shock you are going to use. I had sidluxe originally M/M tune, had to add larger token and make compression stiffer. It was bottoming out really easy on bumpy road, xc riding(im max 76kg)

I just installed the Neco headset, and it's so much better.  Spcycle should use these as stock.  I will probably replace it with the Acros when it comes it as it looks like an even better design, and more robust bearings, but honestly, it's just a bike geek upgrade, this neco bearing does all the things at 1/3 the price.

One thing that I had to order that I didn't plan out:  the rear shock bushings!  Bikes are so different that there isn't really a standard size, for the 686: Front 8x50mm, Rear 8x22mm.  And of course a bushing press if you dont have one.  Neither item is terribly expensive, but nearly impossible to work around.

You will need a cable fish line to get the rear brake line and shifting cable through, which you should expect with interally routed anything.

Consider upgrading the bolts/hardware, they are fine, but a bit prone to rust.  I haven't done so yet, but thinking of it.  I need to find a source for the parts.  I might just paint the exposed metal as a solution.

I didn't have any issues with the brake surfacing on the mounts, everything went together smoothly there. 

Consider the grease you are using to put things together, instead of bike grease I am using Mobil XHP 222 for the bearings and seals, it's a lot more robust than most geases, and a lot cheaper.  For suspension bits, and anything moving a faster speeds I'm using a calcium sulfonate grease, it is less prone to attack surfaces like some lithium greases can.  I ran across this great youtube video by Mapdec which actually explains quite a lot: https://youtu.be/WqiTtlVo-xA?si=CxEbj9GcMscli1fj. Totally worth a watch before assembling a new build.

I have only ridden about 400km during the fall before the snow and the cold came. But so far so good. I am really happy with how it performs on the rough stuff. I haven't dialed the rear shock settings fully yet. And it has not even been ridden in the correct geo, since I had a 100mm travelk fork from start.
I am coming from the DH and Enduro side so I ride the bike fairly hard on descents. There are no really big descents where I live at the moment but I think it feels pretty solid on the tech. The leverage ratio on a bike like this with pedal effecieny in mind (I hope) will of course be a bit more "bumpy" than a more trail oriented bike. I think I will get an even better feel for the bike once the dropper post is in place. Always weird to ride fast downhill with that high seatpost