Thanks to Zlotan and Menno for generous feedback!
Problem solved, it seems I've just been overloading, resulting in all kinds of capacity issues internally.
I've rigged the ingester/eGPU again, and now it seems to do the job far better.
Just for sharing, in case somebody should be interested:
I'm trying to find an all right solution for Mac, that I can tour with two decent cameras in hand luggage (about 10 kg's) for live input to projector image.
This is what I've come up with so far, and now things seems to be running nicely (8-9 layers in 1080/50P, doing 40-55fps):
2x Canon Legria GX10 cameras. Veeery nice form factor for touring. They have some lag, about 120ms. More if I keep the streadyshot on, of course. These are identical to the "pro" version XF405, except pro audio, SDI out and ethernet control. I tested.
I ingest with a Decklink mini 4K for now, in a Sonnet Echo Express SE1 (has ThB 3) It works fine, with around 50-60ms latency on my system, but I need two camera inputs, so I'm looking forward to getting the new Decklink Quad HDMI sometime next week. - It should fit in the box and get enough power (I hope)
Then I have a 2016 Macbook Pro, top spec's. It works fine, and Resolume Arena runs very stable. BUT the GPU sucks. Of course.
So I fell in love with the Sonnet Puck with the AMD 570 card eGPU card in it (also ThB3). But the power supply is big and heavy almost as a brick, I kid you not.
I monitor everything with the Activity Monitor (in utilities), bring up the "Show GPU history". The e-GPU works at 100% capacity in Resolume, it's amazing:) And the two internal cards support. (As an ugly contrast is Adobe Premiere Pro, which I make my living on - That one only uses 30-50%, argh)
A tip if you want to use the eGPU without external monitor. The little freeware program called "Set eGPU" will let you use the eGPU for rendering and other stuff internally https://egpu.io/forums/mac-setup/potent ... s-10-13-4/