How to improve from 1500 to 1800-900 as a busy adult


Reaching 1800-1900 with only an hour or so of chess each day is going to be very difficult. I crossed that level years ago, but I was investing many hours into chess per day. That being said, one hour per day can still add up quickly and you can see from your one hour per day progress that improvement is still possible; it will just take much longer to make progress.
If you can't invest more time into chess, then go with what you've got and just make the best of it.

I never quite understood people asking “how to gain 300 rating points “ etc. Do you like playing chess? So just play it! I don’t know what is the point of being 300 points higher rated. But if you want an advice, do more puzzles. 1200 in puzzles is pretty poor for 1500 rapid player. For example one friend of mine is 1400 rapid and 2400 in puzzles.

Aha, you are non premium so that’s why you solve only 3 puzzles a day. Anyway, everyone who wants to improve has to analyse every game and try to remember his mistakes before he starts another game.
If you have plateaued and still want to improve, then you need to do something different.
instead of simply playing one hour a day, you will need to combine study with practice.
if you have already tried studying, then you will have to study something different.
What/How to study? That depends on what works for you and what you have already done.
One suggestion is to go through 60 or so of your losses. Try to identify the cause of each loss. Time pressure? Unfamiliar opening? Tactical blunder? Etc. Perhaps there is a pattern. If so, that tells you what to work on.
Another suggestion is to attend an OTB club. See if you can find a stronger player who can go over your games. This type of instruction is far more helpful than random advice you get in a forum.
i also recommend books in which the writer discusses a master’s thought process during a game. You can read a bunch of books and see which authors resonate with you.
Good luck.