I would be grateful if you could recommend a woocommerce theme for a very large site incl approx 20k categories, in 3 languages (at first) and.... approx 1.3m skus (no variations on these, so simple)
Also if you could recommend freelancers/developers that are experience to develop such projects.
WP Intense looks great by the way
Themify or Studiopress themes are all very fast at scale. There are others that are good – the main issues arise normally with some theme features – e.g. a lot of themes include an option to change the on-sale flag to a % discount and they all seem to have copy/pasted the same slow code from somewhere.
Rehub is also good. Storefront is bad but not unfixable – they basically use vanilla WooCommerce shortcodes for best selling products and others on their home page and 404 page which kill your scalability once you grow. You can create a child theme and remove these shortcodes however.
As for developers, the main difference with a large site is in the importing area. You should find someone experienced with WP All Import probably.