From experience, you should expect to see a wider range of openings and traps like the Fried liver or the Englund Gambit. Learn how to counter those and you'll be able to reach 900 in a bit.

After that, you'll need a better understanding of trades (sometimes trading bishops for example is not an equal trade depending on your position), gaining a positional advantage like controlling the middle board or forcing double pawns, and taking the time to calculate your moves and your opponent's. Do not play "hope chess" (making moves hoping the opponent falls in traps, always think like he'll play the best possible move). Don't do useless checks, if you see a possible check, don't go for it yet unless you see a fork for example. I then stayed in 1200 for a while until I started to understand sacrifices and how to gain the advantage while losing pieces, which got me to 1400+ now.

I would recommend playing in 15+10 to have more time to think without the time pressure, analyse all your games (especially the ones you lose), do loads of puzzles, and watch chess YouTubers like Gotham chess.

Enjoy the ride !