I'd say that chess fundamentals is three things: Endgames, tactics, and positional strategy. OP's strategy of studying openings and tactics is a very fun and accessible improvement path for intelligent new players, but it is very fragile, as you become vulnerable the second the opponent gets you out of your opening theory. Studying endgames and positional motifs gives you important decision-making tools in unfamiliar positions. Hiring a chess coach is probably the easiest way to systematically improve in these areas, if you're not a robot immune to the tedium of working through Dvoretsky's endgame manual and Silman's Reassess your Chess.
After getting a handle on the fundamentals, the next step is just the accumulation of ideas. GMs use this word all the time in lectures and their post-mortem interviews. Some are common and obvious -- pressuring f2/f7, or yoloing a pawn storm in oppositely castled positions, or outposting an "octopus knight" on the sixth rank, for example. Other ideas require so much genius to see they become famous -- Fischer's Nh4!! at age 13, or Short's king walk, or Shirov's bishop sacrifice, for example.
Accumulating ideas is why studying openings can be helpful in the beginning -- you will learn common plans as well as the most dangerous ideas and traps by brute force just by looking at enough theory. But rather than this inefficient approach -- since you'll never remember every single possible move -- I would recommend studying books and lectures that cover common ideas in the setups you prefer. Specifically, work through the pawn structures you like from GM Mauricio Flores Rios's Chess Structures book, and then study grandmasters who match your style or otherwise inspires you in some way -- e.g. Fischer/Tal for tactical wizards, Karpov/Kramnik for positional specialists, Carlsen/Capablanca for endgame grinders, or Rapport/Jobava/Larsen if you are a weirdo -- and watch Youtube videos analyzing their games and/or buy a book with GM commentary of their best hits.