I installed a horrible theme (DEEP Digital) for my web design biz and have had no search rankings in 8 months. I am an SEO wizard, my on-page SEO is great but the technical /backend of website sucks. I paid another developer to do speed optimizations to fix my serp mishaps but there still a ton of bloated code weighing down my chances of getting included on Google. After much thought I am moving the website to the Hello theme. I also had a very long domain/business name so bought a simple domain name -not taking any chances with Google thinking its still that bad domain. I have enough design experience since launching that I will recreate the bought (bloated code) theme visually from the Elementor Hello theme. But I am not experienced with migration of content. Can I install the new theme and work from that on current WordPress website then point my new domain name or will the leftover unminified code and unused JS from the old theme files remain on that wordpress install once I swap the theme? I am thinking to just start from scratch on new domain with a fresh install of Hello theme and copy content manually from each webpage to the new website domain. Then I will remove the old website so Google dosnt see the duplicate content. Which way is best? Many thanks for your help.

Best way to migrate would be to create a new WordPress instance and move content page by page on the new format. Once you finish building all your pages you migrate that site over to replace the current one.
Elementor is just as bad with bloated code. If you are trying to lean up I would probably go the default block editor route. Find an FSE/Blocks theme such as Spectra, Blocksy, or any other.
If you need more complex functionality maybe start thinking about a page builder such as Bricks.
It depends on the website you have and how much you are trying to migrate.
If it’s informational and built with elementor, just install a light theme like Hello Elementor and migrate page by page.
Let us know how it goes and feel free to reach out if you get stuck!
What is the URL. Maybe we can check what is the issue