Which options are really effective for self-publishing a book?

A friend of mine has published on Amazon. And I think he is having a bit of success with this. But he is very good at marketing. I think that's what it all comes down to.

I quite agree marketing is essential, even a fab book won't sell if people don't know about it, and ideally a lesser known topic, there are lots of romance books for example, and less on social work fiction, my area.

MsLexia, Jericho Writers and Writers HQ is a very helpful independent publishers- does that help at all.

As a writer with four published books under my belt I would just say this.
Who do you write for? Yourself or to make money?
If it is the former self publishing is fine, though some regard this as a form of vanity publishing.
You may want a traditional contract, but bear in mind these are hard to get without an agent. They will edit, proofread etc and be invaluable with publicity. However you will not receive royalties until your advance is paid in full.
Vanity publishers just take your book, spelling and grammar mistakes and all. And for a price publish it.
And then there are the hybrid companies, who proofread, and publicise, etc for a fee. Royalties are yours immediately.
Research a lot …but don’t always believe bad reports. These often come from people whose manuscript has been rejected
by a company.It’s a mine field…so good luck.
Keep us informed as to what you choose to do.🤞