I am far from that rating, so I will not say anything specific. I am sure that a stronger player will give you some useful input. Universal advice for any rating is to determine your weak points and work on them. I've seen videos where the advice is not to jump too much from one aspect to the other, but to focus on one thing for a few weeks at least as it is more effective, and try to implement it in games before moving on.
If you are truly 1 800 FIDE, you surely have to start studying specific opening lines. Many advanced players use Chessbase, but it is not free so I am not sure if that is the option for you. There are of course free alternatives out there.
As for endgames, Dvoretsky's endgame manual is great for the advanced players, or so I've heard. It is too advanced for beginner and intermediate players. But if endgames are your weak point, you might want to go through some easier books first (100 endgames you must know by Jesus de la Villa and Silman's complete endgame manual). Those 2 are the most popular introductory books to Dvoretsky, or so I've heard, I don't have the experience with those books. I can't specifically say if Dvoretsky is too tough for you at the time, but if you've never worked on the endgame, then you should probably consider working through some of those easier books first.
I will leave recommendations on strategy and positional play to someone more suitable than me. I take it that you already go through master games in the openings you play at least. If not (but I presume you do if you are 1 800 OTB), that should be a good way to learn some strategic ideas for the said opening, and maybe some thematic tactical shots that occur from time to time.
If all of this is obvious, sorry for wasting your time. 
From 1800 Elo to 2100 Elo (OTB Rating)
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Mar 10, 2021
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Hello,
I play chess for two years now. I have improved my game play from 1200 elo to 1800 in 16 Months. I play two or three long online games per week and I also trained very much tactics. In the last months I just train a bit tactics. Maybe 2 hours per week, sometimes I train more.
I wanted to ask how you reached an elo of 2100. There are few aspects which I rarely train. For instance opening, endgames and positional play. I just train tactics, play online long games and that's it. My goal is to reach elo 2000-2100 in nine months.
How would you try it (aspects and books)?