How many hours a day do you have available to play? Because that changes the strategy quite a bit. What a lot of people don't mention is that Switch Online has two versions: the basic one (pretty cheap) and the Expansion Pack (more expensive), and it's not always worth it depending on what you're looking for. The basic version gives you access to some classic NES and SNES games, but honestly, if what you want is modern entertainment without breaking the bank, there are better options.
The indie games around the 5-15 euro range are brutal in terms of how much content you get. Stuff like Hollow Knight, Celeste, Into the Breach... they give you dozens of hours. But here's what others overlook: some "cheap" games on sale are fake deals, like marked down 10% when they normally cost the same. Check the price history carefully before buying, use sites that track eShop discounts. And watch out, some free games (like Fortnite or Pokémon Unite) might seem cool at first but then they ask you for money to progress properly.
Switch Online is only worth it if you're feeling nostalgic or want to play online with friends in specific games. If not, spend those euros on a couple of good indies. I'd say look for the weekly eShop sales - there's always surprises there - and don't get caught up in the hype of buying new AAA games just because they exist. Start with one or two while you save up for the rest.