What should I do in this case ?

I have a website very well ranked (it took 1-2 years).
The website url is [keyword].com

I plan to create a second website which treats a topic in the same theme. And I know that I will do other websites in the future with other topics but always in the same theme.

I could create a website for each, and every time something like [keyword].com and just put links around to the other sites.

Or I could try to move the first site "[keyword].com" to "[global].com" (by hopefully keeping the previous ranking) and then to create all other topics related directly in [global].com

Have someone ever gone through this ? What would your advice be ?

Thank you

How did you rank the website? Have you done link building manually or bought links from BHW marketplace?

Personally I'd create new sites. I have about a dozen in one niche created over about ten years, some rise, some fall back but always have some decent traffic. Even when they fall back they're useful for links and supporting the other sites.
But I guess you could be more successful putting all your efforts into one site though. Bigger risk/reward concentrating on one.