Hello there!

I've been meditating on and off, but mostly off, for the past five years. However now I'm trying to really be a lot more focused and actually make legitimate progress instead of just relaxing and getting the occasional bit of clarity. (I have experienced a few interesting things here and there, too, but never in a consistent way).
Currently I meditate for 30 minutes every morning. I'm trying to increase by 1 minute a day until I get to 60 minutes, but my knees are objecting so I'll either have to get a new posture or just be patient with my body. I currently sit in Seiza, with dyhani mudra, and the tip of my tongue touching the roof of my mouth, which I learned from a Zen center.
My breathing might be a little unusual. What I do is I breathe into my belly (well, into my lungs, but I guess this pushes down my diaphragm and makes my belly go out), but I want a full breath. In order to do this, I continue to inhale, but then my chest fills and my belly goes back in. This creates a slight tingling in my spine and it often cracks. I think this might be energy going up, but it never seems to pass my neck. I pause for a second, and then on the exhale I just relax until the end, when I force the breath out just a little bit. I try and make it a smooth process, though.
It seems to be going ok. My attention definitely becomes more 'subtle' in that I become very sensitive to what's happening in my body. Sometimes I get a bit blissful but nothing intense. But I get confused as to where I'm supposed to put my attention because at that point the "breath" seems like such a broad topic. I'll try to focus on the tip of my nose, but then I can't help feeling my belly go in and out. Or I'll try and focus on the whole motion, but then it feels like I'm not focusing but rather simply paying attention to whatever breath related sensations I have. It seems like after about 15 minutes I'm in a pretty good place, but by 20 minutes I might lose that and wonder if I'm doing it right, and by 30 minutes my knees are saying, "dude.. you're done." I could just sit in a chair, but I really do like the way seiza keeps my back quite straight with minimal effort.
When I'm done, I do a bit of homemade yoga, which means I just do what my body feels like it wants to do. This usually involves going into child's pose for a minute, and there's actually quite a bit of heat coming off my spine. This tells me that yes, something probably actually is happening energetically. And then a few other stretches just cuz they feel good.
Anyway, if you're reading this, thank you! Do you have any tips for me? I really would like to move forward.