SEO ranking cannot be guaranteed

Hi there, I have a client who has many of her pages and blog posts ranking high. She is in a very narrow niche with low competition but good volume. She wants to create a job board and the plugins do not meet all her requirements. So we decided to create a separate WordPress install just for job board. I did find a job board theme that meets her requirements and she wants it to rank as fast as possible. I did set the expectation that SEO rankings cannot be guaranteed by a specific month. She has a job page on her main domain which ranks high in google search and she wants that to be replaced with a job board on either a sub-domain or a different domain. She does not want to blog on the job board website. It`s purely for reading and posting jobs. She blogs regularly on her main domain. What would be best practice for this: 1. Point her high ranking `curated JOB LISTINGS` page to sub-domain on the main domain and add it in Google Search Console? 2. Point her ranking page to a separate domain? 3. Or some other option? Thanks in advance.