Hi there
I'm Nowshad Hossain, a MEAN Stack, WP Plugin, and Extension developer. I can migrate your extension to MV3, and I have experience doing this before. You can check my reviews.
The amount and time is placeholder since I don't know how many files you have in your extension, cause if there's so much of code, then after migrations, we need to make some changes to them too to work on MV3.
Will be waiting for your response, thanks