Hi @koen ----- Thank you for the follow up 
In regard to your questions…
“The only thing that still puzzles me a bit is your remark about my collection and/or the device. Is my 14k album collection (too) big for Roon? I thought I read on the forum that people with 50k collections have no performance problems at all.”
Please allow me to offer some further insight here, and apologies for any confusion my comment about your collection size may have caused. As mentioned, with 1.3 quite a bit of work has been put into further enhancing the overall user experience, and one area that should see these benefits is our handling of larger collections.
As Brian has very eloquently pointed out in that thread:
“That said, there are definitely some areas where Roon could be optimized, and some specific performance oriented projects are planned. We are just wrapping up a round of optimization on RAAT–our next scheduled bit of performance work is focused on improving library related performance, with a focus on mac and linux, which should help here.”
"Would I gain speed by replacing its default power cable by e.g. an iFi iPower? "
Replacing the power cable for your storage device will not increase it’s speed or capability of handling media.
Would things improve if I take the Mac Mini i7 upstairs, together with the WD HDD, install RoonServer on it, and take the older Mac Mini downstairs to run RoonBridge on it, connecting both Macs with an ethernet cable? And does this mean I have no GUI downstairs (RoonBridge) and upstairs (RoonServer), requiring the purchase of a device (iPad/iPhone) to use as a remote? Or is there a way to still have some GUI to control and edit my library with just RoonServer and RoonBridge?
If you have the option of a stronger machine running your core it is never a bad idea 
I would honestly consider running Roon Server on that device and running Roon Remote on the older Mac.
-Eric