Advice on migrating a website to WP.

I have a very simple website with a page for each of the 4 services offered by a photographer. Currently this site is on a platform called Alboom. Recently I have been having problems with the platform because it is "rigged". The site presents pages whose canonicals point to different links. I can't register H1, H2 tags. I am only free to change the text and image on the website, anything beyond that I need to open a ticket with support to change it on the website and sometimes this support denies it. Anyway, some impediments that frustrate me in trying to improve the site's SEO. So I built an updated version of this site on Wordpress, much more beautiful, lighter, organized and optimized and I intend to migrate while keeping the same domain and "SEO history" However, I have never done a site migration. So I would like to hear some advice. I already know that I should start by backing up the website, but what would be the next steps?