Can't install Sonoma, external nvme drive not selectable

I have a mid 2017 iMac 5k (18,3) running Ventura from an external nvme drive.

Trying to update to Sonoma. Followed the latest guides, but once in boating up and in de installation dialog I can't select my current Ventura installation. It's the right format file system. Any tips to get beyond this step? I don't want to do a compleet clean install, too much hassle for everything that is currently setup within Ventura.

Hope there are some tips. Thanks!