This is a relatively short list for a reason: at least as of version 7, Vivaldi's default settings are reasonably secure, private AND ergonomic. Note that the settings available for the mobile versions of Vivaldi differ considerably from those in the desktop versions.
Also keep in mind that I'm currently using four extensions with Vivaldi on my desktops: uBlock Origin Lite, Bitwarden password manager, Raindrop bookmark manager, and Dark Reader to apply dark mode to web pages.
Only my changes to the defaults appear below.
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General > Homepage > Specific Page > your favorite homepage
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General > Startup with > Homepage
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Appearance > Window Appeaance > User Interface Density > Compact (rectangular rather than rounded tabs)
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Appearance > Status Bar > Hide Status Bar
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Themes > Dark
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Start Page > Start Page Navigation > Hide
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Tabs > Tab Bar Position > Top
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Tabs > New Tab Page > Homepage
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Boonarks > Bookmark Bar > Uncheck Show Bookmark Bar
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Search > Default Search Engine > Your Favorite Search Engine
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Search > Private Window Search Engine > Your Favorite Search Engine
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Privacy and Security > Tracking Prevention > Ask Websites Not to Track Me
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Privacy and Security > Tracker and Ad Blocking > No Blocking
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Privacy and Security > Passwords > Uncheck Save Webpage Passwords
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Privacy and Security > Payment Methods > Unckeck Save and Fill Passwords
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Privacy and Security > Payment Methods > Unckeck Allow SItes to Check for Saved Payment Methods
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Privacy and Security > Addresses and More > Uncheck Save and Fill Addresses
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Third-Party Cookies > Block in Incognito Mode Only