The final draft is the beta reader stage. Everything should be as close as possible to being done.
Can I Self-Publish at This Stage?
After it, yes. Enough sets of eyeballs should have seen it to make sure it's as error-free and ready to publish as humanly possible, but the publishing process (even as a self-publisher) is a whole other set of processes after this. So, I would hold off.
What does this stage do?
This stage is the last stage before sending it off to agents or publishers or considering self-publishing. It's the last stage of drafting. After this, you have a completed manuscript that could (if an agent or editor at a publishing house says so) lead to more drafting and edits, but you took the time to give that editor something that shows you're a professional.
If my family were to find a manuscript after all of these stages were done and allowed the time to be done properly, I wouldn't be embarrassed about what they've found.
The manuscript has been checked thoroughly to meet the following standards:
Plot holes and inconsistent details (such as a character's eyes changing colour, or going from one hair colour to another without a logical explanation).
That any plot devices or tech referenced works the way it should.
Geography is consistent.
The writing style has been tweaked, and tugged, and honed to make every single paragraph, sentence and word have maximum impact.
As error-free grammatically and spelling-wise as humanly possible with no missing words or unfortunate typos.
Punctuation is used correctly and where it needs to be.
It's the last flight check before sending it off.
After this, it's done. Open a bottle of champagne and pour a glass. Celebrate this milestone that probably took a few years or at least a few months.
Print it one last, final time in this form.
And let it rest one more time before sending it off. Read it with fresh eyes.
If absolutely nothing jumps out at you...
... It's time to move to the publishing process.