from 1200 to 2100 on chess.com: some pieces of advice and anecdotes

Nice post @Armand_Spenser, and nice progress! I've had a similar journey, moving up from around 625 in both rapid and blitz time controls 3 years ago to 2100+ in both time controls now.

The only thing I'd say differently, more so to the benefit of readers, is that contrary to common advice, I personally did see improvement by playing blitz almost exclusively.
Less than 10% of my total games have been Rapid, and over the past year, probably less than 2% have been. Despite this, I have seen steady growth. 
My study routine since around 1700 has largely come from tactical work, with a bias for tricky openings and dubious side-lines, standard tactics exercises, playing lots of blitz and then analysing my games for weaknesses afterwards and finally, ALOT of hours on YouTube to digest thoughts and advice from stronger players than myself.

Definitely like your tip about -25 / +400 for your opponent seeks, I did the same thing myself. You'll always learn more from losing to a stronger player than thrashing a weaker player!

Thanks so much!

@Kraig What are your thoughts on Puzzle Rush?

Puzzle rush has its place. Most people will say it's just for fun, but I'd also suggest it's great for improving your pattern recognition and shallow calculation (good for blitz) by virtue of exposing you to multiple quick tactics back to back.

I started doing it pretty much daily since asking and yeah not only is it fun but I really felt it has improved my visualisation, calculation and pattern recognition a lot, it feels even more helpful than rated puzzles to me. I’m sure it would be immensely beneficial to complete beginners as well who would learn in no time the potential of their activated pieces working together.