What is a little bit worrying in your case is that your rapid rating never reached 1550. 1500 blitz rating maps to 1650 rapid rating. On the other had, 1200 blitz rating maps to 1400 rapid rating and your rapid rating is in the 1400s (and your rapid rating was in the 1400s when your blitz rating was in the 1500s which is very unusual). If you start to play more rapid games, spend more time thinking about the moves, watch chess content on youtube, analyse your games after you play them, you will get your rapid rating to 1600-1700 and then your blitz rating will naturally get to 1500 when you start playing blitz again. You can still play a little bit of blitz here and there but focusing on a little bit longer games may be what you need since you have played 20k blitz games and that didn't really lead you anywhere. In general, many people don't advise playing blitz for improvement and instead they advise focusing on slower time controls.
This.
If you play a lot you'll get a lucky win streak now and then. Especially if you rematch (someone on tilt loses to you over and over) and you get a really high peak rating... but your real rating is 100s of points lower.
The only thing to be scared of is if you looked at a lot of different user's graphs and they were ____ rating for a long time (time and games), and then dropped.
He is a peculiar case as his rapid rating never crossed 1550 even though he played rapid games when his blitz rating was 1500. 1500 blitz rating maps to around 1650 rapid. His highest blitz rating of 1555 maps to 1700 rapid (150 points higher than what his highest rapid rating was). After this info I start to wonder how he had a 1500 blitz rating in the first place. By the way, here is the data for comparing blitz and rapid ratings https://www.chessratingcomparison.com/Graphs