Gweilo354 the following actually works in a stable fashion for me;
https://start.duckduckgo.com/
Top right 3-lines menu > Settings (on DDG page, not the browser settings)
here do all settings, disable any AI features, return to the settings when saved at bottom of the page.
You'll see that the ddg-settings has four separate sub pages,
https://imgur.com/a/xrJXE3Y
go through them all and complete your configurations, save. Make sure to allow 'GET-Requests' in the 'Privacy' sub-category.
Now, at bottom of ddg-settings, tap 'Save bookmarklet and settings data', longpress the weirdly coded "Bookmarklet"-address and save it, all your settings is encoded in this address and will remain a fast way for you to restore your Ddg-settings in the case you've lost them.
Next, do a simple search from the configured https://start.duckduckgo.com/, say, search for just a letter, keep pressing the search icon to make the search multiple times.
Check in Vanadium settings if https://start.duckduckgo.com has been addable as new default. Eventually it should show up.