… what can install between the starter battery, and the lithium battery bank, to stop it from going over 13.8 volts, but still get a decent amount of amps while driving?¹⁾

Given that:

Alternator charging methods combine the batteries only at certain times:

  by 12v trigger.  Example:  12v source that's only active when the key is in the RUN position.  These have 4 lugs:  starting+, house+, trigger+, trigger-; or, 
  by voltage sensing.  These usually have 3 lugs:  starting+ in, house+ out, starting-.  The starting- "ground" is only used to complete a small circuit so the relay can run itself.  So it's usually very thin.  

If you disrupt (or “ground”) the relay cannot turn on, cannot the batteries.

So-o-o-o-o-o…..

You can:

Use a high voltage disconnect on the house side set to something very safe (13.5V?) that would disrupt the trigger (or ground) circuit.

Example: you start out the morning with house batteries at 12.5v or something. Drive to McDonalds for coffee and free wifi.

On the way the alternator brings the house bank up to 13.5V. The HVD disrupts the trigger (or ground) and the relay/isolator turns off.

Caveat: I am currently doing this on my van at 13.8V even though I have FLA batteries. I do this to keep solar from pushing 14.7v into the chassis while driving if the batteries happen to be in Absorption.