MPPT 150 | 35 trouble getting consistent wattage w/ F3800 Anker

I’m looking to output a constant voltage to charge an Anker f3800. I’ve got a 150/35’s and I think I’m just not understanding the voltage settings to get this working right.

I am not sure of the voltage across the battery +/- input. The f3800 is built to take sub 60v PV directly in with no charge controller, so it’s not what the mppt is looking for when it looks at battery voltage. The 150/30 seems to fall into

I’ve got 3x Sunpower X21-460-470-COM panels (77v 6a panels) into the mppt, then into the f3800. What would be the best way to keep the controller in bulk charging mode all of the time? Max Batt voltage setting is 48, but absorbtion/float can be set at 60v. I can get it to hold at 58v for a while, but It always seems to fall to around 16v 20a (I have max input set at 20a). I’m thinking it could be either:
A) non-representatice voltage being read at the ‘battert’ because it’s not a normal raw battery, causing the voltage to drop. Is there a way around this? Anyone modded FW to get around this?
B) For some reason my panels are maxing out at around 320 watts, which doesn’t seem right. I’ve been able to get 1000w out of them with the sun directly above. MAYBE a cabling issue?

Let me know if you have any ideas… Open to whatever at this point. I’ll do the happy solar dance if that’s what it takes.

Edit: could I just get a Smart Battery Sense unit and show it a specific voltage constantly, and tell the 150/35 to use that as Batt voltage? Could this fool it?