I am pretty sure you will love it.
The Black trails in Scotland are a step up on the Welsh ones but that doesn’t make them not impossible.
Broadly speaking at Glentress the black is just longer than the red and not significantly more difficult.
At Dalbeattie and Kirroughtree as well as being longer there are some challenging sections that you may prefer to look at rather than just riding straight into at full tilt. There are usually routes around if you don’t fancy them.
Also on the subject of Kirroughtree you get onto the black at the most distant part of the red, it adds probably 1 hour maybe 90 minutes so you may as well do it all in one hit.
Further north you have the trails at Laggan Wolftrax, Golspie, Fort Wiliam, Balblair, Learnie and Morray. These again have some challenging sections and Learnie inparticular has some big surprises that really should be looked at before you ride them.
Have fun.