Store bought cactus/citrus soil has worked well in my experience in larger scales. It's good, really available, and serves as a nice low maintenance soil for periods when it just needs to leaf out. Not that heavy. Generalist stuff.
I will apply your tips in my next generation of bonsai clones. Small amounts of the big bag soil really fluctuates unpredictably in terms of moisture content.
Yeah that sounds like it will work. Rooted cuttings probably need no fertilizer at all. Just keep it covered with a cup or humidity dome if you can. Same with a graft, keep it tightly wrapped in plastic.