I personally would set it up in Google Search Console for each subdomain and experiment to see how deep Google matches subdomains on subdomains.

And do 302 redirects (https://first.steps.for.planning.wedding ==> https://planning.wedding/first-steps)

Although I'm not sure what I'd do on https://for.planning.wedding, but probably a 302 to https://planning.wedding

The nerd in me would then automate any slug that ends in "planning wedding", set up the domain in Search Console and ensure its verified properly.

For the record, I really like this branding idea.