Separate a subsite from a main site

Hello

I am following up from another post I made https://social.technet.microsoft.com/Forums/office/en-US/7a0e8137-296e-4a8a-8719-163ab012cb09/migration-from-2013-to-2019-databasesetup-questions

We have a site that had a development on SharePoint where they made some changes and installed Dlls and such. Now we need to migrate to 2016 and then to 2019 and I fear the subsite they developed will not work properly. Is it possible to migrate an entire site, and leave the subsite behind by splitting it or do something similar? Having the developers migrate it is not possible at the moment.

I am thinking of these scenarios:

1. If posible move-spsite to remove the subsite and switch it to another database, then migrate the site to another server and leave it behind.
2. Migrate the entire site to 2016 and just delete the problematic subsite (might bloat the DB) and then delete all the subsites from 2013 and have that behind (the DB might need some serious shrinking)
   
3. If move-spsite is not possible, somehow split the subsite with some other command.
   

Do you have any ideas that might be good? Or perhaps would it be better to use some sort of migration tool?

Thanks!