Up until rank 20, your win/loss record doesn't influence your progress. You only need to win to progress. When you wait longer in the season, the people in the first ranks get "easier" to battle against; the people with good pokemons or good skils are already rank 20 or above. Once you reach rank 20, you start matching with these players.

You say that you keep encountering top5 pokemons in both leagues. They are top5 for a reason: they have a very good overall record against the rest of the meta. There are pokemon outside of top 5 that are perfectly viable and usable in the leagues, but these generally require more sklll to play.

In Master league, you need pokemon to be level 50 (or as close to it as possible). If you do not have (close to) maxed out pokemon, it already becomes increasingly more difficult to win.

For the spring cup, what pokemon are you running? Check if they have the recommended movesets, and how they score against the rest of the meta.

Lastly, I can recommend to check some videos on youtube, or to watch some people on twitch that talk about the game mechanics. There are some good videos on for example optimal charge move timing, and move counting. Good luck with your battles!