I have purchased seven templates the last in 2019.
How soon will there be Joomla 4 versions and will they be at additional cost?

I feel maybe the change to Joomla 4 would be a good time to redo all the sites from scratch with new Joomla 4 compatable templates.

Would you advise that approach as may actually be easier and give better result?. Its a lot of work but the change to Joomla 4 looks easire that way.