800 blitz, but they have a rapid of less than 600 and most of the time played 20,000 games....
800 blitz is loaded with (Engine use) Opinion.

If they are rated 800 and cheating, then they are bad at chess and bad at cheating as well.
Take ownership for your losses and for your mistakes.
You lose because you make mistakes, not because of some conspiracy to cheat against you.

I understand your situation. Try to keep calm and it's not a big deal if you don't improve in blitz, many are there like you and they are even worse.

It might be a case of playing a system that is well known with 1 horrible blunder. Some games play themselevs with easier to see moves.

If you suspect someone, report them.
https://support.chess.com/article/209-how-do-i-report-someone
https://support.chess.com/article/648-what-do-i-need-to-know-about-fair-play-on-chess-com
https://www.chess.com/article/view/online-chess-cheating
https://www.chess.com/article/view/chesscom-update-march-2024#FairPlay
The site closed 57,000 accounts last month for fair play violations and has a full team of staff working on reports along with some automated systems for detection.
https://www.chess.com/article/view/fair-play
That said, discussions of cheating, potential cheating, or cheat detection are not allowed in the general forums. If you would like to discuss join the following club. https://www.chess.com/club/cheating-forum
I keep coming up against players who are playing near perfect games with three pronged attacks and no blunders. This keeps occurring at the crest of crossing into a higher rank I was at 900 blitz then faced 6 perfect opponents which knocked my rating down now as I try to gain back the eilo again same stuff my last game was a perfect example the player is playing incredibly, but their rapid is 600, but somehow playing blitz at 1300 to 1500 eilo? If you played long enough you can feel the play level this is what I keep seeing and I'm so tired of losing in this fashion that maybe its best to just leave this site. My opinion. Losing is fine, but when it feels and looks wrong it probably is.