Hello
Sorry for bad English. I'm working on it. I want to offer my services for this project.
Last two years I have been working on shopware.
I have experience with database migration Shopware 4 to 5
I also do database migration from shopware 4 to 5 like a sub-shop. It is more difficult.
If now the site is not actively used, I can just disable it in the time and do the update.
Otherwise, the work will need to be divided into 2 stages:
first (dev website):
1) Site transferred to a test server
2) Make a complete update of all plugins
3) Do update the shop kernel.
4) You and I will test everything to make sure that everything works
second (live website):
1) We disable the site (you might want to do it at night when there are no buyers)
2) We repeat all the same thing they were doing on the dev site, only this time on the live site.
3) if all is well , then we turn on a new website.
4) If something goes wrong, we can always restore the old site from a backup and to solve the problem.
Making update in 2 stages may seem silly. But unfortunately, on shopware, with a large number of plugins, many of which have an encoded code, the update can occur don't properly, and this we just did not see on the first time. ... Therefore, we should not hurry to test everything.
You can use a template from shopware 4 on 5. But I do not recommend it, because many functions will not work. It is also possible, we need a specialist in html / css