That’s tough to answer, not knowing your setup. I’ve never brewed any other way but BIAB so I can’t really answer the first question, but I suppose that would be a reasonable solution.
As far as sparging/rinsing the brew bag after mash, in my experience I don’t think that will help your efficiency much, as opposed to just full volume mashing from the start. What I have found is that a thinner mash leads to better efficiency/extraction (whatever the proper term is). So in my kettle, if I mash thick and sparge I *might* only end up where I would have if I had just mashed in full volume to begin with. Maybe less. That’s a bit confusing for sure, and maybe that only applies to my crush/pot/set up. With the crush you are working with, for BIAB, I would probably get better extraction if I mashed with a larger volume of water, didn’t sparge, and just boiled longer. Again, that’s only one brewer’s opinion.
But really, I would try and find a way to crush your grains finer. Run them through a pulse on a blender if you have to. It’s very difficult to crush too fine for BIAB.
Good luck. Please keep us posted.