I think you made a good choice with the poly. It maintains the color of the wood well while an oil like tung or linseed will darken it. If there is some nice figuring it may be worth using the linseed oil though. Oils also take more skill to apply well and you should do tests on scrap pieces first. I also wanted to mention that you should try making a router sled for flattening slabs like this. they are extremely easy to make and use and you end up with a perfectly flat slab in no time which just requires a little finish sanding. I use this bit https://www.amazon.com/gp/product/B01N2UQETH/ref=oh_aui_detailpage_o02_s00?ie=UTF8&psc=1 Also NICE WORK!!! thanks for posting some actual woodwork required planning, nice wood, skilled tool use and good finish work. The construction pine stuff gets old on here
> uxcell 1/2-Inch Shank 2-Inch Cutting Dia Double Flute Carbide Tipped Cleaning Bottom Router Bit Cutter