From my own experience (as a publisher, not a software developer) it is MUCH less stress and mess to leave your existing site on WordPress. Move it to a subdomain, if possible. Then load Bolt and start your site fresh. Add a link to the subdomain for the old site and content.
I have not found any good way to migrate WordPress over to anything else. I spent a lot of time trying to do exactly that last year, 2017. I found b2evolution had a pretty reliable import for WordPress, but even that had problems due to the WordPress import files. Bigger files were more likely to have problems. One of the biggest problems were image files no longer being attached to posts. I also had categories which repeated several times, not sure why.
So, if you can leave the old content online, with WordPress, you will save yourself a lot of time and stress. If you must move it then the only way is manually, one post at a time. It is time consuming but, you're very likely going to need to re-edit, fix and reattach images to each post anyway.