It's been several years since I welded with AMI's. I have never butted the 37.5 together like you described. Is that normal these days or is it just the company you are with?
Open butt requires a lot of skill when things don't go perfectly, but gets great results when it does.
Inserts are great too, but more fit up time is required.
J prep, square butt (with and without wire) good joint configs.
Lots of options, I just haven't welded with NO gap on conventional bevels.
Your suck back, is it only at the bottom half, bottom third?
I have pulled root passes with straight current (say 110-120 with no background)and like you describe-180/80 or similar.
Arc volts must be VERY tight on the problem area.
Also, you may try starting at 1:00-2:00 DH progression first and get the bottom over with early before the metal gets too hot(pinching action) and causes more suck back.
Can you leave a slight gap in the bottom (1/16") ? That will help.
Make sure that you have NO lag in the torch assembly.
Try a little more blunt on your tungsten. If that doesn't help try no blunt.
Wire entry at leading edge of puddle, no dripping into the puddle. The wire should help push the root in.
Not a good joint config IMO
Any hi-lo on fit up will wreak havoc on ID.
Sorry that I can't offer more help but that is not a typical J/C in my experience.
I miss working with the machines, AMI, Dimetrics etc.