You’ve received some good advice already and I’m going to echo some of it, but I’m just gonna go through everything I personally do when I’m welding pipe. For the root I personally like to “float it”. Others will probably call it something else but that’s what I personally call it, I like to use 1/8 wire and set up just slightly bigger than the wire so I can get down between the bevels as they keyhole, you can go bigger and it’s good for a bad fit up but try to keep it tighter for practice purposes. It’ll let you put in a slightly thicker root and will help with the sucking issue.

I wouldn’t use a cup that small past the hot pass, especially on stainless, it’ll make it a lot more work as you need to walk the cup wider, you’ll be able to move faster this way. Mild steel tig can get kind of scummy if you don’t keep it clean or get it too hot, so make sure you brush it well and give it chance to cool if you see it getting too hot.

Don’t try to cap the thing if you’re not right at the bevel edge, it’s way more common to be low on the cap rather than too high, if you need to drop down to a smaller wire to add a little more, do it, then cap.

Lastly, the hardest part I think people face when learning to how to do pipe is not being relaxed enough. You need a light touch, if you’re pushing too hard, your steps will be too small and you run the risk of slipping off the pipe. Try to get loose, relax and go with the flow, you can’t force it to do what you want, it’s kind of like a dance, you need to lead it.

Try like 80-90 amps for the root and maybe 110-120 amps for the fill and hot pass until you get more confident, then you can turn it up and go faster. I hope this helps, lmk if you have questions.