18 years of experience in the industry here.
What do you mean by an 'adult site'? Adult incorporates a whole host of niches. Cams, adult dating, porn videos, etc. What exact kind of 'adult site' do you mean.
My honest advice would be don't bother, unless you understand the adult niche very well.
Building any site up to a million uvs a month (in any niche) is extremely difficult. These are post-HCU days in which ranking for keywords organically is very difficult.
I know many people who have owned adult sites for over 15 years, who don't even get 1 million visitors a month. So you are setting yourself a mountain to climb.
If you have a pot to spend, it might be better just buying an existing site. We have exxisting adult sites for sale, which you can see in my link below.
But my honest advice, knowing this industry pretty much inside out, is don't bother, unless you have a VERY unique selling point.
Well, I agree that you should understand how to do things.
But the problem is you're scaring people to advertise your service calling things "extremely hard".
Do you know what is extremely hard? Risking your life or scaling 10 companies at the same time.
If your dumb sites are "extremely hard", I don't know what is building companies. Super ultra extremely mega hard?
Now, get out.
The problem why it is so hard is that 99% of times it's trash like that site you linked.
yes, honestly, put your hard-earned cash towards something else.

If you do have $5k to invest in the adult niche, Adult dating is e a much better idea. You can launch a white-label site (I run several) for next to nothing, by partnering with the big white label providers. You will need to probably buy traffic to your site, as organic rankings are extremely tough, but that can still be very profitable. in terms of ROI.
You don't even know how to make a decent site. What are you talking about?
You have weak marketing angles to turn this thread into your sales thread.
It's not 2005 when you had a domain from 1999 and you could rank under 10 million keywords.
Times have changed and anyone who thinks some domain names are the only thing that matters, should visit a doctor and tell them about living in the past and that their past is influencing them in a negative way.
Like people think that some domain name justifies everything they do. It's like those instagramers that got 2m followers but dance naked in front of camera and think their follower count justifies their trash.
---
My suggestion is to not rank adult sites cause it's:
1. Shitty niche which pays shitty money.
2. It's not worth ranking black hat keywords.
3. You simply do a large site and grab some traffic from all around the internet.
4. You don't focus on single keywords, but traffic in general.
That's it. You don't need to do any SEO if you only need traffic.
Even if you reach that 1m visitors monthly, there's still no guarantee you'd make decent money.

If you can't automate it fully, don't touch it. It's what black hat is. Full automation or don't play.
You'd have to build a PBN, probably automate that too which most people can't even think about.
With black hat niches you do 99% of yourself. There's nobody that'll provide decent links to these niches, unless it's just negative ROI investment like "$1000 per link" lol.
The thing is that you build PBN, it dies after some time. And you need to repeat that process constantly.
It's like super hard to play it long term.
Focus on finding your target audience and what they need, instead of looking at some nonsense like videos where the only people think about is imagery. You want to understand "why am I doing x and y" instead of following some random advice blindly.
Don't make choices based on impressions of other people and opinions. Follow facts that you deduce yourself creating hypotheses in your own google docs. Write down 10 pages and other possibilities. Analyze opportunities. Use tools similar to SWOT analysis and see pros / cons of multiple choices.