How to solidify melted chocolate?

I am scared that the chocolate will stick to the counter top (in plastic) or the sheet pan.

Don't do it on the counter top. The best surface would be a silicone mat from a baking store - a smooth one, not the ones that seal an woven steel wool inside and so have a relief pattern. If you don't (yet) have that, plastic foil will do, you'll be able to peel it from the chocolate when cooled.

I am scared that it won't fully solidify. I am wondering whether the counter top should be cooled down (which is impossible in my kitchen).

20 C is a good temperature for cooling chocolate. Never cool the counter top. If anything, it would be best to warm it slightly so the bottom of the chocolate bark is cooled slower than its top, this gives the best shine. You can skip that though, it's a perfectionist's thing. If you are interested in a deeper explanation, see How to prevent "sweating" chocolate covered strawberries?.

I am scared that it could be hard to warm up the chocolate at a high enough temperature without burning it so that it is easy to spread smoothly.

Here comes the most important part. The problem is not even burning, the problem is distempering. The chocolate will melt at around 30 Celsius, but will distemper at around 33 Celsius. Distempered chocolate solidifies in a grainy mass. You should not let it go warmer than 33 C, better yet 32, and that's a very narrow temperature range to hit.

To avoid this, you have to work in small batches¹, use a double boiler for warming up, and use a candy thermometer. Don't use the microwave, that's uncontrollable and only useful if you are going to make some kind of chocolate mixture, not if you want to resolidify it as bars or similar. Also see What is the best way to melt chocolate? and What is the purpose of tempering chocolate?.

_{¹ At the beginning, "small" means "so little you can stomach tossing it" because you will probably get it wrong a few times, it is a skill you have to build. But not so little that it smears onto the bowl and cannot be handled, I'd say the minimum is somewhere between 25 and 50 g if you are using a small enough bowl. Later, when you are confident in your process, "small" means "as much that you don't get a heat gradient in the melted chocolate in the double boiler", it depends somewhat on geometry and bowl material, and you have to experiment with that a bit to find out what amount works well.}

It sounds like you are making a 'bark'. When you said you had a 'large amount' of chocolate, I'm hoping you mean ounces and not kilos. When I melt 16 ounces, it takes about two minutes in the microwave. Zap it in 30 second chunks. Each time, removing the bowl to stir. Be super careful. Once it is creamy, it will seize or burn easily. Alternately, you can place a metal bowl over a sauce pan of simmering water. The chocolate melts slower and you have more control but you risk moisture getting into the chocolate from the steam. Experiment with little amounts. It will build your confidence while not risking waste. Parchment paper on a cookie sheet or just free form on a table will work. I have never had it stick. Great idea. So many possibilities for flavor combos. This should be fun:)

Every year at my mother's house we make chocolate & pecan candies (look for enjambres de nuez in Google), 2 o 3kg at times.

To melt chocolate without burning, melt it in Bain Marie, is slow, but safe.

Use paper wax in your sheet pan to prevent the chocolate from sticking.

And one "grandma" trick: if you want the chocolate to solidify quick, add a very little amount of liquor to the melted chocolate, maybe one tbsp for pound. We use Brandy, it doesn't change the flavor. If you add a lot of liquor, the chocolate will solidify really fast and also the flavor will change. I don't know the exact chemical process involved in this, but it works :).