Yeast DOES matter. I like dry yeast like Cote des Blancs or US-05, these two will provide consistently great results. I did not get good results from Nottingham or S-04. I have also had good success with the Wyeast 4184 sweet mead yeast and champagne yeast, although both of these resulted in an extremely dry result. Cote des Blancs will also be quite dry, just a hint of sweetness left. US-05 will leave a little residual sweetness as well as a hint of honey-like beery flavor that is pleasant and not at all bad.
FYI – A Belgian yeast used in cider will probably NOT taste distinctively Belgian. It just tastes like normal cider. You will likely NOT get the distinctive esters or phenols. In my experience it ferments fairly clean like many other yeasts. But it will definitely work, so if you have some fresh stuff on hand, by all means go ahead and try it.
I assume you mean you will add 1/2 Campden tablet per gallon? If only 1/2 for the entire 5 gallons, it wouldn’t be near enough to have any effect at all. In fact you really should use at least 1 full tablet per gallon, if not 1.5. I wouldn’t go up to 2.
Chaptalization I think is a good idea as long as it’s not overboard. 1/2 pound is fine for 5 gallons. 5 pounds, on the other hand for example, would be crazy high in alcohol and might also finish sweet if the yeast crapped out early from the high octane. I usually do not chaptalize but if I do, it is a small amount like 1/2 pound or so to add just a few points of gravity but not a lot. You can add it up front since yes, the sugars are all simple.
I have tried to ferment low and slow with varying success. The first batch I made this fall got down to SG=0.997 in just 4 days! And that was at 62 F with Cote des Blancs. I did NOT add any Campden to this batch, which was probably my downfall. When I got surprised by this, I immediately killed the yeast with 2 tablets per gallon AND I added gelatin and cold crashed to knock out the yeast completely. Obviously, the cider is now finished fermenting and clear as crystal. But in the past, using 1 Campden per gallon, fermentation has been much slower paced, and if you want to slow fermentation at any point, combine racking with smaller Campden additions of perhaps 1/2 Campden per gallon, which won’t kill the yeast but will slow them down a bit, and add gelatin if necessary to slow things down further. Even gelatin doesn’t stop the yeast entirely but just makes it that much harder for them to have sex and whatnot. In any case, cool temperatures in the 50s are probably best. 60s are okay but still a little too warm in my experience. Treat your cider fermentations sort of like a lager – low and slow indeed, with additional chemical controls to slow things down as needed.
I like your idea about freezing some cider for backsweetening. I am going to try adding a can of apple juice concentrate per 5 gallons along with Campden and sorbate to see how that works. I have had varying success with varying sweetening methods in the past, sometimes successful but most typically I have ended up with overcarbonation and an extremely dry cider when fermentation decides to take off again in the bottles. I have not kegged my cider in the past although I might have to give that a try this year. I have several 5-liter kegs which would be just perfect for the small 1-gallon batches I am making. I have made 2 batches so far and will make another 1 or 2 in the next couple of weeks. I’m trying different apple blends this year, keeping the yeast and fermentation conditions all the same, for comparison purposes. Last couple years I played with yeast. Good old trusty Cote des Blancs has been my favorite for years, but US-05 turned out very nice as well.
Seems you are very much on the right path, Pietro. Best of luck to you.