My recommendation at this point is to go into the .htaccess file and replace everything there with the standard WordPress .htaccess rules only. No redirects or any other configuration changes you may have attempted there.
Then I would just go ahead with the website migration and not worry about the redirect.
If you do want to setup a redirect, I would recommend to do that through Cloudflare instead. That way, you don’t have to worry about WordPress and redirect code interfering with each other, as well as it being a little bit more performant. But again, in this case, I would not bother.
.htaccess code is applied immediately, no need to wait for anything. However, please do keep in mind that:
- There was some syntactically invalid .htaccess code at the top of the file. You’ll need to fix or remove that too to solve the 404 error problem.
- Cache in your browser may make it seem like changes are not taking effect. You can bypass that by trying to access the page through an Incognito window in your browser.