I’ve been doing woodworking for over 30 years now and also have made a lot of cutting boards and restored a couple as well. One thing I’d recommend on the underside with the cracks is to fill those w/ titebond III wood glue and fine sawdust. Rub the glue into the crack so that they are good and filled and then rub the fine dust into the crack or just run the sander over those glued areas and the sanding dust will fill the cracks on their own. You can also pre mix the glue w/ the sawdust so that it’s like a wood filler. Then you let dry for a day or two, sand off and continue where you left off. It’s recommended when making a cutting board or gluing boards for any top to wait 7-10 days after glue up for the glue to “cure”. This also prevents “glue creep” from occurring where the boards can slightly move causing the joints to be able to be felt when you run your hands over the boards.