I could do a fresh install of a version 2.3.5, or 2.3.7 or whatever (using another db-prefix).
Take version 2.3.7 for a clean install.
Then, I could take a backup from my currently running 2.3.4 forum and restore it in the newly installed version. That would avoid using the update procedure.
Import a backup of your database content, not of your forum. And take the change in the settings table, described in my reply to Micha, into account (second last paragraph of the posting).
Of course, I would have to adapt look and feel of the new version, but in total it might be less trouble.
Your idea looks promising. I don't know, which and how many changes you made in the theme. During the development of the 2.4-branch we introduced several changes to the default theme, namely security relevant fields in most of the forms. We have a changelog for the default theme starting with version 2.4.6. It shows not every change since 2.3.4 but it nevertheless might be helpful.
Trenne niemals Müll, denn er hat nur eine Silbe!