Building site in advance when domain is already active on wordpress.com

My site, https://www.cityoftongues.com, is currently running on wordpress.com. I’ve organized hosting and installed wordpress on a host so I can transfer over to www.remarpro.com, but I’m anxious about how to manage the transition, in particular the question of what happens after I change the nameservers.

What I’d like to do is rebuild my site on my new host in advance, then import all my posts etc from wordpress.com, so that it’s sitting there ready to run as soon as the nameservers change over.

The problem is that I don’t seem to be able to log in to www.remarpro.com UNTIL the nameservers have changed. That means I can’t build the site in advance. And since the change to the nameservers may take a few hours I can’t predict in advance when I’ll be able to start building it, and I run a very real risk of my site being in total disarray while I scramble to get it all uploaded and ready.

This seems like a really obvious problem, so I’m assuming there must be a really obvious solution. The only problem is I can’t work out what it is. How do I go about building a version of the site on my host while the wordpress.com site is running? Any advice would be very much appreciated.