Problems after working on 42re transmission

Hey everyone. So, I don't peek my head out too often on these forums, mostly I try to read them. But I've never seen anything referring to my current situation. Which tell me that it's either because the answer is completely obvious or the question is just really dumb. But I'm just baffled to say the least & my Chilton's isn't sitting the answer out about this one.
Ok, so 04 GC Limited 4.0l, straight 6, 42re tranny. Been having issues with the tranny since we bought it. If it's cold it will , sometimes slip but comes right back. We've had it 5 years, it has always come back. I check/change fluids often as I travel for work & I need it reliable. Last week it started getting much worse, slipping here and there but also shifting harder. I pulled over fairly soon after feeling this, checked the fluid & there were bubbles in it. I panicked but I had to get to work so I babied it there when I got off the next morning I was trying to baby it again & sure enough... Limp mode. I
have been wanting to replace the shift solenoid anyway. So I got that, replaced it along with the accumulater spring (which was broken in 4 pieces.)
Put in as much transmission fluid as I took out (4qts.... Am I wrong to think that's enough? Because after letting it warm up upon checking, it had nothing in it??? That's my first question... Where did it all go? No leaks that I can see, plus when I drive, my coolant gauge starts to go down as well as my coolant level.
2nd question, did I ruin both my oil cooler and tranny, or could it possibly be just the hoses? I know I should do the torque converter at the same time, I just had no funds to do all that, until today. Is it worth all that when I found a rebuild with 20,000 miles for $350?
I've really been contemplating getting something else, just don't want the payment, but I'm willing to do so & turn this into a project...
Any thoughts, advice or even any 'what the hell were you thinking('s) would be welcome. I don't want to walk away from this without at least learning what happened. This was the first time I've been brave enough to venture into the tranny past the filter change, I love and I hate to say that it gave me confidence to go back. If we end up getting this rebuild I will definitely be tearing this bad boy apart if just to learn from it, but would love to try my hand at a rebuild.
Please feel free to comment... Help me figure this out of you will. I'd love to banter about it.
Thanks so much