I dunno why the AT-15A is off limits for ya, I've matched my earnings in my IS-6 at times and the thing can be damn-near invincible.
But let's see... The Panther/M10 at 7 is a fun one, makes me okay credits. I just had a match in it where got 25k credits with full provisions 2.2k damage, while spamming APCR. Nothing makes nearly/easily as much as the IS-6 though. IS-6 at 8 is a given (although I've been using the CDC more since I got it, it makes money better, I think.). I'd gander I make ~60-70k in a decent game in the IS-6 (I'm gonna guess-timate that it'll take 2k-ish damage). Regardless of damage I usually stay out of the red, so the IS-6 is most likely your answer. However, the IS-6 will spoil you and will make a bad tanker worse. You gotta know how to drive a tank before all else, know how to angle, side-scrape, hull down, etc., before you jump in it, but know that as far as skill goes, at tier 8 you'll still deal with idiot teams. Teams are still crap and carrying is harder than at earlier tiers. If you end up getting the Is-6, don't be the one that hides behind Meds/TD's. Be the heavy and put in work for your team.
I can't speak to the effectiveness of the Angry Connor but I hear it might be a good alternative.
A lesson learned at 3001 battles: Be nice. Not because you have to, but because you can't "unsay" things.
And then you've got to live with yourself. So be kind, for you and everyone around you.