As a general rule never make more that 1 change at a time to a web site. Let me explain.
You are changing the domain name. So only change that, do not change the URL structure. If you change the domain name and the URL structure at the same time and your rankings take a dive, you have to figure out which change caused the drop.
This is what you need to do....
1. Make a backup of your old site.
2. Restore the backup to the new domain.
3. On the old site put this redirect in place. I am assuming this is a linux server... this also sends the visitor to the https version of the new site.
RewriteEngine On
RewriteCond %{HTTP_HOST} ^domain.com$ [OR]
RewriteCond %{HTTP_HOST} ^www.domain.com$
RewriteRule ^(.*)$ https://newdomain.com$1 [R=301,L]
Now this preserve your rankings and also preserves any backlinks, and takes very little time to accomplish.
Now wait for a week just to make sure you have no issues, normally you won't. This gives you time to switch over you Google Analytics and GSC to the new properties and start recording your statistics.
Now if you want to change the theme or content or URLs then you can do so and if you rankings drop you will know what caused it.