Beneden alle peil

If all we are probing with current observations is BHs formed since z~1 the mass growth is limited. I fully agree it is a big stretch, but I would like to find even better arguments.

I’m not sure. Binary BH formation could be inefficient at early times and the LVK systems all originated relatively recently. The GW only probe a tiny fraction of the mass in BH, so it is easier to argue around it I would think

Problem with the examples so far (as far as I can see) is that they are small numbers, so there is wiggle room (exotic formation, observational bias etc). I'm quite sure this cannot be true, but we need one (or more) killer argument that this simply cannot be true...

Also, all stellar BH formed early in galaxies would become quite massive. A quick calculation for the Milky Way shows that the O(10^7) BHs would have masses up to 5000Msun with total mass 2.3e10Msun! I thought it would be dead easy to rule that out, but haven't found a killer argument yet...