Something I said but not gone through in detail.
90% of the overheating problems I've noticed, heard and experienced is the circulation of coolant. So a rebleed isn't completely out of the question.
OP IMO make sure it was bled correctly:
reso cap off
car running
heat on full blast
fill reso till coolant starts to climb
immediately replace cap
shut car off
turn screw on thermostat housing to bleed out air bubbles,
Repeat till no air bubbles come out
I think you stated there's no oil /coolant mixing going on. That's a good thing.
If there's no coolant in the oil or vise versa usually that would rule out a BHG.
However that's not to say it's not possible.
Flushing the radiator separate will insure you get all that 20+ year old crap out of the radiator. For if it's the original one. I can almost guarantee it's got some crap in it.
Don't use a flush. Just water from a water hose. Hit all three holes... hehehe. But cover one so some type of pressure is there.
The flush solvents eat at the aluminum.
Doing this saves you from having to buy a new one. I did this twice on two different e34s. Both times there was all types of mess it there.
IIRC the WP would make some type of noise signalling it's about to fail. (at least mine did)
As well here's something else to consider. This is M30 specific.
Let's say the coolant has been bled correctly, wp is okay, no BHG, and the radiator has zero blockage.
(something you wouldn't know unless you removed and flushed the radiator through all 3 holes)
It may be your sensors. There are 3:
2 in the thermostat housing
one in the radiator.
The one in the radiator and iirc the blue sensor on the thermostat housing are the most common to go. These regulate the temp and would mimic a overheating issue.
Before I left atlanta. I replaced my radiator and thermostat for the ride north.
The car read death hot. Being I knew I had no issues with the WP, nor did I have a BHG.
I chaulked it up to the sensors for I nor the the PO changed them out.
Once I changed them out the car went back to normal.
just my .02. Back to work. Lunch is over