Well you will need content for 10k keywords ofcourse.

Publishing content isn't enough you have to build links in order to get them ranked. You can use PBN's although its risky but if you do it right then you can get ranked for which ever keyword you want. Or, you can hire some content writers for you who can build content and guest post in your niche blogs for links.

You need to do SEO and SMO, build a quality link building on relevant category and also content for all keywords.

attach yoast plugin in your wordpress ,then you should have to create backlinks with different sites(its depend your client where they want to drop traffic) and its take time almost 6 months to increase your traffic on your keyword as your client given you

Its ecommerece website and virtually every category and product pages are your content and keywords. You should focus on things like how to improve product pages and category pages. Strong on-page SEO with rich snippet. Then build overall domain authority, that will help every other pages to rise in ranking. Instead of building links, you should try earning the links. How? Tell people who bought products from your store to spread word about you on their blogs, websites or social media profiles.

SEO point of view first you should know website stretcher. Proper tags , Friendly URL, Alt tags , Meta Data and much more than you start promote site on Social Media, Social Book Marking, Guest blogging and off-site articles and much more but step by step. And bofore on-site select few keywords. For more detail P.M

I assume that they have some combination of product categories and products.

First break up sitemaps into multiple files. One for product pages, one for category pages, one for support and brand pages, and one for editorial pages like blog posts, buying guides, lists etc.

You need to see if your page types have issues apart from each other.

Next... adopt 1 keyword per page. use the best product keywords on product pages, use the best category keywords on category pages. Focus brand keywords on home and support pages. Everything else should probably happen in the blog or in custom landing pages for email, social, PPC, and promotions.

You can make the blog a separate website that cross links with the products it promotes. It doesn't have to be integrated. Its up to you on that.

Doing it this way will make it easy to target thousands of terms in appropriate places and in appropriate ways.