Short answer: before writing a single page, define: 1 pillar page, 6 to 10 clusters with their long-tail keyword, the interlinking between them, and the hosting/subdomain it will live on. This template repeats identically for every new niche in the catalog.
If the niche will live as a subdomain under the main hub, you need hosting that makes creating subdomains easy. Affiliate link: I may earn a commission if you purchase through this link. Hostinger is an accessible option for this — it allows unlimited subdomains on most of its plans, which fits well with a catalog that will keep growing into new niches over time.
Define the niche's core topic and the broad keyword it targets. This page will receive links from every cluster, so it needs to be the most complete page in the niche.
Each cluster targets a specific long-tail keyword identified using the SEO pillar's method. You don't need all 10 ready before launching — 3-4 solid ones are enough to start, adding the rest over the following weeks.
Each cluster links to the pillar using the primary keyword as anchor text, and laterally to 1-2 related clusters in the same niche.