Advise on updating the site and blog on Wordpress

There is an outdated html site with a little Javascript and a Wordpress blog that we also want to update.
Now we have made another site on the test Wordpress, we want to run it instead of the existing one.

Nothing has been done with the blog yet due to the fact that:
We use a lot of add-ons. They are either constantly updated or can be
generally abandoned. Updating the Wordpress itself, on which the blog is fraught with scrapping, for the reason that the scanner shows
incompatibility of our custom theme with
Need professional advice on Wordpress (without any work on the site) to clarify the following issues:

how to update the blog in our particular casehow to combine a new test site with a new or updated blogkid blog themes can be used
is it possible to embody the idea of a dynamic main page of the blog, how to bring posts to the main page without changing the date of publication (change their order, without problems group by topic, etc.)
how to transfer all our articles to a new blog without losses (there are about 400 of them)how not to harm the SEO of the site and blog when launching a new site and blogcan all the ideas on the site and blog be embodied on one Wordpress
how to save all URLs, etc.The cost of consultations is discussed