I baste on the floor using the yellow Dritz quilting pins and Pinmoors. There are several cheap Pinmoor alternatives online. I'm not a fan of heavy quilting either, my hubby hates what he calls "the fabric cracker" lol. I'd do SITD with a walking foot but just do the ditches that you want
I use Warm Company Soft & Bright poly batting primarily and it's a ten inch apart batting like yours. I don't think I've ever quilted it that far apart but I don't think it would be a problem. I've made several blankets for children that I quilted six inches apart and they've held up to heavy use wonderfully. definitely make a test sandwich to see how small you want your stiches, etc. I also highly recommend a calming drink like a cup of tea or cocoa or an adult beverage and some nice music
have fun!!