A long time ago I was an almost-competitive bodybuilder (I got really sick from overtraining in preparation for what was to be my first contest back in my college days - I'm 42 now - and decided I just didn't want to go thorugh the hassle of contest prep). I still keep up with it as a hobby.
I'd say definitely stick with whatever routine for at least 6, preferably at least 12, weeks at a time. Switching routines is a must, but IMO you shouldn't do it until your body stops responding to what you're already doing - no more frequently than every few months, and maybe only a couple of times a year. There's the HITT method, German Volume Training, and everything in between. The trick is to find out what your body responds to the best and for that you've got to give each method a chance to see results - which means several months.
IMO you're getting plenty of rest. Split routines are the norm in bodybuilding. No need to sit on your butt every other day if you give the muscles you last worked a break of several days. You may in fact need to try to work each bodypart twice a week instead of just once, but your routine tells me you're on the right track - working opposing muscle groups, split routine.
You might want to go for a 3-on, 1-off split. Take that last workout out and work your shoulders with the bis and tris and your calves with the rest of your legs. That way you'll hit every bodypart twice every 8 days. Try cardio in the mornings and lift in the afternoon or evening, if you can. Cardio on two of your 3 lifting days. Probably skip cardio on leg day and the day after if you use your legs a lot doing it, as most folks do.
Nutrition is often overlooked, so I would check that closely too.
There's far more to it all than I can post here. I HIGHLY recommend you check out the articles, and most especially read the forums, at
Bodybuilding.com. From there you can go lots of other places, but personally I think they're a GREAT place to start.
P.S. Crunches are a must, and will build big abdominal muscles too. But the muscles will not become visible until you cut the fat, and for that you've got to do cardio. Though I would bulk up first since that seems to be your main concern, then cut.