Hi Pedro007,
you can just do a "Drag and Drop" from your Macintosh HD ( Internal ) to your External SSD.
May be, you should have to re-install some Apps ( not those furnished with Big Sur ).
It's a long time procedure but the best after a clean intall. By doing like that You generally avoid the copy of bad system files in your new and clean SSD.
I've had many problem with an iMac 2017 and I used this way to get all my files and documents.
Have you done a Time Machine Backup?
GGERARD