Agile, a series of Waterfalls
I have been through many different ways to manage development of software during my career. I have cowboy coded and ninja-deployed. I have had months of research and spec writing. I have had 2, 3, 4 and 5 week sprints. I have had daily standups at the beginning of the day, at the end of the day, once a week, once a month. I had retrospectives at the end of a sprint, at the end of a month, at the end of a quarter, at the end of a feature, at the end of a release. I have estimated in days, story points, US Dollars, T-Shirt sizes. I have done Waterfall, Kanban, Scrum, Lean, XP, Waterboarding. I think I've even been a Rockstar developer once, by which I mean that I threw up after drinking too much and speaking sim-lish.
In short: If you can imagine a way to develop software, I've likely experienced it first hand. And I am still astounded how much the term "Agile" (not to be confused with the similar Italian word) gets abused. So please, let me start by giving the complete, unabridged, definitive definition of Agile Development:
We are uncovering better ways of developing software by doing it and helping others do it. Through this work we have come to value:
Agile Manifesto, https://agilemanifesto.org/
Individuals and interactions over processes and tools
Working software over comprehensive documentation
Customer collaboration over contract negotiation
Responding to change over following a plan
That is, while there is value in the items on the right, we value the items on the left more.
This is it. This is the entirety of the Agile Manifesto. And yet, this short, 4-clause manifesto has spawned thousands and thousands of pages of written books, coaching and courses, a plethora of new vocabulary, and of course, certifications and TLA galore. PMI-ACP, CSM, CSPO, CSD, CSP, PSM, PSPO, PSD-I, SAFe, MFG, OMGWTFBBQGTFO, Six Sigma.
Now, this stuff is good for the job market. A lot of Project/Product/Program Managers (or "Producers" in the Entertainment industry - but if you ask 10 people what that term actually means, you get 15 different answers) managed to make a career out of this, and I'm not against that. Technology is changing really fast, and that affects non-developers as well.
But it also causes a paradox: Too many times, it leads to an attempt to force "Agile Development" to follow extremely specific, narrow rules. Otherwise, you are doing it "wrong". The counter-argument to this is of course that not having any rules means that you're just winging it.
The point about Agile Development - and software development in general - is that you need to figure out what works for your team and your kind of work. Are you writing software for government/medical/military use? There's gonna be waterfall-levels of spec writing involved, no way around that. But you can still split out features that are specced, develop them in sprints and regularly check with the customer. Are you a startup that's sustained on Ramen, a shoestring budget that doesn't have any room to actually buy shoestrings? Just hacking stuff together and deploying it 73 times an hour might be A-OK. Are you writing corporate software that requires early heads-up? Having monthly or quarterly releases and doing "3 week waterfalls" is an option.
The two worst ways I've seen software development handled is trying to conform to very strict rules that clearly don't work for the kind of work you're doing, and just constantly changing the process under the guise of making adjustments, but really just hoping that something just magically works.
But there's no silver bullet, and there's really no way around for the person or people in charge of the team/process to understand software development to the point of making educated adjustments. If your process isn't working - why is that? That's a tricky question to answer, and often one that stays unanswered as random adjustments are made.
Figure out if it's a problem with your customer interaction (More/Less frequent reviews? Involvement in the sprint planning or not? Approving every change before even starting real work on it, or making decisions and asking for approval during the process?), if it's a problem with your team structure (Too few developers doing too many features at once? Too many developers working on one feature? Skill sets not matching up with tasks?), if it's a problem with your decision making process (Does it take 3 weeks to even get a meeting about whether the button should be on the left or right of the form, without even making a decision? Unless that button is the "Test Missile Alert" button, you should probably look at a faster way to make decisions.) Do you have one super-urgent "do or die" feature that needs to go in NOW? Abandon parts of the process that stand in the way of implementing the feature.
Everything is a tool. While it's perfectly possible to make a career without knowing the tools available to you, it's worth learning the pros/cons/strengths/weaknesses of each available tool. That book or certification you got? It's a tool to put on your CV to land a job, but it's also a way to learn more about tools.
Use only that which works, and take it from any place you can find it.