Welcome! I'm so glad you found this site...it's a wonderful place with fantastic videos to help you and the people on this forum are amazing.
Good for you for figuring out a way to cast on that won't create too tight stitches to start. As far as when you get a couple rows into your stitching and it is too tight...are you tightening each stitch after making it? That can lead to really tight stitches. Typically, the first and last stitch are the most vulnerable to being too loose so those are more prone to needing a bit of gentle tightening but the rest simply depends on how you are holding your working yarn. There are tons of ways you can hold it...I find that the easiest for me is to loop it around my pinky finger up over the back of the fourth and third finger and then around front of my second finger. But there are many other ways too - it's simply what gives you the most comfort AND control. I'd play around with being sure NOT to tighten each stitch and how you hold the yarn and I bet your stitches will come out a bit looser. Also, be sure the size needles you have and the yarn you are using are appropriate for each other - that can help too.
Good luck and let us know how your stitching is coming along! :muah: