How big will some of the largest mega super massive black hole get during the black hole era..?

I've seen studies that indicate that the actual mass of a black hole does not have an upper limit. You are right - the more massive the black hole is, the less critical tidal forces become over short distances, but your question at the end - how dense a black hole would be - is relatively easily answered...

I'm sure you know this - the black hole is actually the singularity at the center of the event horizon; The event horizon itself is an indication to how much *mass* the black hole has - the wider the EH is, the more massive the singularity is. That being said, the more massive the black hole, the denser the singularity seems to be as well (Think of Jupiter and Saturn - Jupiter is larger, and more massive... If you had two black holes, *size doesn't matter* - they're both very small points in space, and common measurement would indicate they're the *same* size - so, one must be more dense than the other... The only indication of mass we see is the larger EH.)

You can't include the Event Horizon in your measurement of density of the black hole... It's an *effect* of the black hole, but not part of it per se.... and the larger the mass at the center, the lesser the delta in tidal effects tend to be at and beyond the Event Horizon. However- as you get closer and closer to the Black hole itself - the tidal effects *do* start to be an issue - and, it's an inverse square proportion to distance from the black hole, and how strong the tidal effects are.

I have heard this reasoning before.

But it appears not to be borne out by observation.

Very large black holes will ultimately have a mass equal to two or three trillion solar masses.

That will happen when a galaxy cluster undergoes mergers with all galaxies being merged into one. As they merge, their central SMBHs (Super Massive Black Hole) go into to orbit about each other, losing gravitational energy, finally merging.

As they spiral toward each other, the stars of the merging galaxies (especially the stars near the cores of each galaxy), are absorbed by one or other of the SMBH, thus forming an accretion disc.

What we do see is active galaxies where this merging of SMBHs is evidently happening. The energy output from the accretion disc EXCEEDS the total solar energy output by fusion reactions of the original galaxies.

That implies that the particles in the accretion disc are moving with relativistic velocity -- 99.99% of light speed. It appears not to be affected by the Schwarzschild radius. It appears the all up mass of the SMBH is all that is required to accelerate infalling material.

I guess it is a paradox. But recall this :

The Schwarzschild radius is the distance from the singularity where the escape velocity is the speed of light. And conversely, it is the speed at which an object falling from an infinite distance will reach the speed of light as it falls toward the singularity.

I rather think that the laws of gravity (the Einstein concept of gravity) may break down in some way in the presence of such a large mass. --- One or more of the 4 fundamental forces may be united in the vicinity. Mind you that is just me speculating. I'm just trying to (among other things) understand HOW jets of material of billions of solar masses can be formed having velocities of near the speed of light. Something VERY weird is surely going on there.