A wooden countertop is a great idea! I've made dozens of them over the years with many species of wood.
With regards to movement, its no big deal. Elongate the screw holes in your cabinets so the top can move and use silicone which will seal off any spills getting into the cabinets.
There is a wide choice of finishes available depending on the look and how the counter will be used (ie food prep directly on the surface which I would not recommend) Waterlox, Lacquer, Epoxy, Mineral oil and beeswax etc. Make sure you seal the underside with at least 2 coats then the top with several and you should be good to go.
Also, use a glue that doesn't have any "creep" Ive seen a lot of folks using products like tb3, tb3 moves and you don't want a glue joint moving. tb1 or epoxy is what you want to use.
Any questions you have feel free to pm me or post them here happy to help
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