Need help choosing the right SEO agency for my domains!

I am undecided wether to hire an onsite full time SEO team or hire an established SEO agency or a freelancer... for several domains that I plan to develop (my niches are fashion, cinema, events, trade)

have no idea how these seo agencies work and i am afraid about how much it will cost me because I am planning to develop 5 domains.

fashion.careers
fashion.deals
cinema.careers
trade.events
movies.vision

the advantage i think is that these domains can interact with eachother and that maybe will help in terms of traffic and content.

I am sure your feedback will help me decide!!