@DerekNIU The sealer was probably the trickiest part of the whole process. There is no perfect solution. The acrylics are easy to apply but will eventually get scratched and may not hold up really well to high heat (meaning placing a hot pot on the surface). There are some polyurethanes that offer more of a "pop" finish and they are more durable, but more complicated to apply. You can't use an epoxy because as I understand it, none of them tolerate UV exposure and will yellow and flake it exposed to direct sunlight.
I ultimately went with the Buddy Rhoads PS1 Reactive Sealer. It's a combination product that includes some concrete hardener (densifier) and surface sealer which I'm pretty sure is an acrylic (they don't say precisely what's in it). Bottom line, I'm expecting that I'll pull out the sander with a 400 grit pad every few years to remove the old sealer and re-apply. I don't plan to try to baby the surface for fear of damaging the sealer because a light sanding and re-application is really a couple hour project at worst. And I built it to use it. I expect it will hold up better than a wood top, but a granite slab would require less maintenance though with a much higher price tag... and it didn't fit with my intent to DIY the whole project.
It is critical to seal it because as you mentioned, concrete will stain but also because since it's such a (relatively) thin slab, if it gets saturated with rain that freezes hard before having the opportunity to dry apparently bad things could happen (micro-fractures, chips and maybe not micro-fractures).
I keep it covered when I'm not using it anyway to keep bird droppings and other gross stuff off of it. For a cover, I simply cut up an old canopy cover I had from an old broken 12x12 canopy. Draped it over the table, cut the excess material, got a cheap grommet kit from Lowes and put a rope through the gromets to lock it in place. Because the material is light, it folds and easily fits in one of the cabinets while I'm using the grill. Not really water proof, just keeps stuff from accumulating on the concrete surface.
Good luck if you decide to build a concrete top. If you do and you want to discuss what to avoid, I can sure help you with some of that...