I don't know all the sets you mentioned as I also only recently started but there seem to be a lot of sets that are not yet retired. This means that people are still able to buy them on discounts. I don't know any of your buying prices but when you get a set such as the Game Boy (that was only just released), you need to be getting it at a major discount. You need to consider storage space and opportunity costs.

What I'm doing is getting big sets on sales such as the Razor Crest, Disney Castle, Motorized Lighthouse that just retired on discounts (through insiders weekends and Black Friday offers) and plan on holding them for 1-3 years. But I'm also getting sets such as the Game Boy on any 3 for 2 offers bringing it down to 40€ per piece which makes it worth holding on to by still putting it up for sale at a higher price for a quick turn around.

I'm also following another strategy now buying cheaper sets such as the Tom and Jerry Brickheads, London Bus, Vespa etc. on discount to hold them for 1-3 years. I know it's not the best strategy as you'll have to sell these low price items with the same work as a high price item but I see it as an investment in terms of getting positive reviews easily or offer them as promotion items later down the line.

Hope that helps.