Adding 10-11 liters worth of marbles(17 liters of beer in 28 liter vessel)? Hmm, that sounds like a lot of marbles to clean, add, then remove later. I would have to go with the idea of no secondary. I have racked 4 gallons in a 5 gallon secondary for 3 weeks and had no oxidizing issue. Don't worry so much about oxidation unless you plan on leaving your beer in the bottle/keg for 6 months or so. My beer does not last more than a few weeks.
I have beersmith and it is good but if you search the internet there are several brewing calculators available that are all pretty good. I personally liked "brewers friend" before I got beersmith.
Mash temps, yeast pitching temps, and ferment temps can be critical so use a decent thermometer. Sanitizing is very important, use good sanitizing practices! Other than that, follow a decent recipe and your beers should come out fine. Oh, be patient, two weeks in primary, three weeks in bottle/keg and most beers come out good!
Keep reading here, there are a lot of good people here with great ideas and most of all, just relax and keep brewing!
“I don't drink beer all the time but I can drink (a) beer anytime" - Me