How can I get to 1500 rating?
Ask your GM Chesscoach

What are some things you could recommend someone in my range to improve their rating?
And for every rating range having coach is always very helpful.

How can I properly do bullet? Whenever I play somewhat slow I take the advantage and get flaged on time, and when I go fast I get mated.

Dag Roeland!
I play London, Caro-Kann and Slav at 800 ELO 20 minute Rapid. I have Chessable courses for all of them, which I've studied, plus many dozens of hours of YouTube videos from GMs that play these openings. I feel like I am pretty good at these openings (I Reviewed all of my 1500-1700 bot games on chesscom).
My problem is that I struggle against "bad" play, where opponents swarm the center with aggressive pawn pushes and/or lock the position with pawns. My pieces seem to have no safe development squares where they normally go. What should I do?
Thanks so much in advance and fijne dag!

That depends on all the things you have studied so far. In general you can never go wrong with a good tactics or endgame book. As for what opening, I would have to take a closer look at your games to see what opening suits you.
Thank you very much ı mostly play blitz and bullet games from mobile app except tournaments and 5+0 tempos when ı am outside or coming from school. My rapid and half of the blitz games are where ı really focused on and played from Laptop. Also ı read like seriously more than 10 chess books. Yasser Seirawan Series most of them, Test your chess IQ August Lishvitz, Alexander Kotov think like a grandmaster, lessons in chess strategy Valeri Bein, Satrançta Ustalık Kursu Selim Gürcan, winning chess Booby Fischer, Systematic training in chess Sergiu Samarian, the fundementals of chess Jose Roul Capablanca. I am sorry if ı wrote one of them wrong ı read all of the books from my own language ı remember.

I’ve been playing chess seriously for the past two years. Most high rated players I have faced have told me that I’m hugely underrated and I was hoping to raise my rating to a more realistic level. Mainly, I have two questions here.
1.) My opponents (600 to 700 ELO) basically have no opening knowledge and they just play whatever they saw GothamChess play one time on a live stream. So should I really expand my opening repertoire? Would it be useful to dip into multiple lines of the QGD if my opponent plays something like 2… h5?
2.) Is the reason I’m finding it difficult to climb the ELO ladder because a lot of other players have the same problem as me. (Basically ELO deflation, where a 600 rating really represents a 1000 rated player??).
Sorry for the super long post. I look forward to your insight! Thanks! :)

Hello GM! I got 1856 in rapid but it decreased back to 1700s. How can I overcome my bad performance?

I don't feel good while playing on playing the semi tarrasch and queens gambit declined and some of the french lines with black. I have a very deep preparation at gucci piannissmo and closed sicilian and chameloon variation opening with white. I always play the exchange variaton of caro cann and french.
For example these blitz games, these type of setups against caro cann and french defense.
Also ı play bad at all bullet tempos and ı even sometimes slow for 3+0 but the thing is ı can play fast but ı generally get confused and make some mistake oviously like most of the players. Because of it ı try to get 10 to 15 sec time advantage and playing fast the opening
What are some things you could recommend someone in my range to improve their rating?
That depends on what your goals are. If you want to get stronger overall, then I some things I would recommend are playing longer time-controls to strengthen your puzzle solving skills, reading a good endgame book and fix any holes in your repertoire you might have.