The OPs code is quite readable and I wouldn't mind maintaining it (vastly better than the real-world legacy PHP code I have to maintain), but my main takeaway is "this is like Python with Java syntax" (interestingly the opposite of Kotlin, i.e. Java with Python syntax).
I'll be the first to admit I have a tendency to C#/Java-ify code in languages that I attempt to learn (could definitely argue it's a bad habit) due to nearly a decade of working with them. Any advice you might have to make things a bit more idiomatic to modern PHP? Always looking to improve the code however I can.
If you're aiming for this state of a codebase you'll lean heavily towards Object/Modular oriented styles which strongly resemble Java/C++ etc - even languages that express them in dramatically different manners (like OCaML) end up looking really familiar when built in a sustainable manner. There is a common visual expectation we have around code that promotes readability[1] and we'll form code in most languages to match that common visual expression. This is actually one reason I decidedly dislike python - since it's visual expression is encoded into the syntax evaluation of the language itself.
Anyways the TL;DR is that, IMO, clean maintainable code looks like clean maintainable code and the language only minorly impacts this appearance.
1. That isn't to say this is necessarily the best visual expression - it could just have been chosen early on and then just ingrained in us all.
There's a lot of this code also written in Python. However, I think Python's forced indentation and cleaner syntax is a boon because it makes bad code look bad and unreadable. Too often, variations in style conventions are used as a smokescreen for rubbish software.