opening theory

I agree that in the initial stages a book will help. The writer will hopefully give insight into the purposes of the moves as well as just giving the moves. For the Polish as White, I found Lapshun and Conticello's "Play 1.b4" (Everyman) useful; or, if you prefer playing it as a St. George for White, Basman's "The New St. George" is invaluable (but rare, out of print, and expensive second-hand). Databases pick up where learning the themes leaves off: one can only evaluate lines when one has sufficient thematic understanding to do so.

Yes!

Your ranking has reached fairly high.

You need to learn Opening Theory & Create a full fledged Repertoire.

Learning Openings Theory is a mixture of Databases & “Coaching”

I put Coaching in quotes because it doesn’t necessarily have to be a literal coach.

What you want is to learn why each move is being played. You could learn the reason from a Coach, Book, Instructor, Blog, or Video etc.

Their is a lot of ways to learn why a move is played which is main part.

Databases tie into learning Opening Theory because ideally you want to learn starting from most popular lines to obscure lines.

Usually, The Main line & Classical lines are the most popular. A good starting point would be with Main Line or Classical Line.

Classical Lines are often written in published books which sometimes was during an era of no engines.

Main lines or something known as Modern lines are often how players today play the line due to the influence of engines.

So yeah if your going to study Theory on 1.b4

You need to start with the Classical & Main lines because chances are your opponent are going to play against you using knowledge they have gotten from books or modern players.

Afterward, you finish with the more popular lines.

You need to find the line which is considered the most challenging or testing.

Every opening has 1 or 2 lines which is considered to be most challenging to deal with. These lines can be very difficult to deal with and you should know how to handle them 100%.

Good Luck on your journey!

Nowdays with leela / databases you have the ability to create your own "theory" in any opening very quickly. In general I've leaned toward avoiding books mainly because they take too long term learn from, and there are very high quality video alternatives to books these days. But if a book focuses on ideas over concrete lines you can still get value out of it, however I'd avoid books that mainly just give concrete lines. But usually I'll start with some hanging pawns videos, then I may buy a course. I do this mainly to get acquainted with the ideas, I don't just follow the prescribes moves in the course. And I'll follow along while looking at the database / engine and considering, based on the stats and eval, different lines I may want to play. Then, at some point... you just have to open up a database / engine / annotated game and flesh out your repertoire. So for the database I use lichess's analysis board with their online game database for rapid/blitz at 2200+ elo... I like this database because the games are online and they're recent. For the engine I use leela which I've configured on my machine, I have a graphics card that will run it... for the annotated game I have chessbase and I actually save all my annotated games to the cloud, so if my PC dies I will still be able to access my repertoire. But I choose each move very carefully... as you're choosing each move you're looking at stats - the frequency and the winrates... you're looking at eval, you're looking at how sharp the line is for you / the opponent, how practical it will be for you to remember it, how often you'll encounter it... and you're trying to consider all the different factors and make a sound decision. And you need to know what your goals are - I'm targeting the repertoire at online 2200ish elo rather than tournament play, this makes a big difference - in any setting where people can prep for you before you'd lean more toward theoretical soundness, but I never really plan on playing in that setting.