Okay. As you've probably guessed, I'm not very familiar with Oracle or its terminology.
Here are the contents of my current Settings:

Privacy protection
	Block ads and surf the web up to three times faster					ON
		Manage exceptions
		Manage lists
	Block trackers														ON
		Manage exceptions
		Manage lists

Wallpapers
	Enable wallpapers													OFF

Privacy and security
	Opera may use web services to improve your browsing experience. You may optionally disable these services.
	Use a prediction service to help complete searches and URLs typed in the address bar
	Site Settings														OFF
		Control what information websites can use and what content they can show you
		More
	Clear browsing data
	Clear history, cookies, cache, and more
	Automatically send crash reports to Opera							OFF
	Help improve Opera by sending feature usage information				OFF
	Protect me from malicious sites										ON
	Fetch images for suggested sources in News, based on history		OFF
	Display promotional notifications									OFF

Appearance
	Enable dark theme													ON
	Show bookmarks bar													OFF
	Font size															Small
	Customize fonts
	Page zoom															100%
	Pressing Tab on a webpage highlights links, as well as form fields	ON

Sidebar
	Manage sidebar

	Synchronization
	Synchronize browsing data with Opera account.						Sign in
	Import bookmarks and settings

Search engine
	Set which search engine is used when searching from the combined search and address bar																	DuckDuckGo
	Manage search engines

Default browser
	Default browser
	Make Opera the default browser										Make default

On startup
		Begin fresh with start page
	X	Retain tabs from previous session
		Display the start page first									OFF
		Open a specific page or set of pages
			Add a new page
			Use current pages

Advanced

Autofill
	Passwords
	Payment methods
	Addresses and more

WebRTC
		Use any suitable network interface (recommended)
		Use default public and private network interfaces only
		Use default public network interfaces only
	X	Disable non-proxied UDP

VPN
	Enable VPN
	Browse with VPN to prevent third parties from tracking you			OFF
	VPN connects to websites via various servers around the world, so your connection speed might be affected

Battery saver
	Enable battery saver												OFF
	Save battery and browse up to 50% longer

My Flow
	My Flow is your personal space for links, videos, images and notes that you want to have on your computer and phone at the same time.
	Enable My Flow														OFF
	Disabling My Flow doesn’t remove any content or connections.

Crypto Wallet
	Connect Opera with your phone to easily make cryptocurrency transactions.
	Enable Crypto Wallet												OFF

Search pop-up
	Enable the search pop-up when selecting text						ON
	On text selection, convert:
	Currencies to USD
	Units
	Time zones

Video pop out
	Enable video pop out												OFF

Personal news
	Check my sources for new content									never
	Show available news feeds on address bar							OFF

Start page
	Hide search box														OFF
	Hide Speed Dial														OFF
		Hide "Add a site" button										OFF
	Receive promoted Speed Dials and Bookmarks							OFF
	Show Speed Dial suggestions											OFF
	Use bigger tiles													OFF
	Adjust Speed Dial animations for slower hardware					OFF
	Automatically adjust the maximum number of columns					OFF
		Maximum number of columns:										3
	Show news on start page												OFF
	Show Continue Shopping section										OFF
	Show Continue on Booking.com section								OFF
	Show weather widget													OFF

User interface
	Show tab previews													OFF
	Enable Workspaces													ON
	Cycle tabs in most recently used order								ON
	Show full URL in combined search and address bar					ON
	Enable search box in the address bar								OFF
	Delay loading of background tabs									ON
	Enable reporting problems with sites								OFF
	Scroll to the top of the page by clicking the active tab			ON
	Warn on closing window with multiple tabs							OFF
	Warn on quitting Opera with multiple tabs open						OFF

Languages
	Language
	Spell check															OFF

Downloads
	Location
	/usr/local/Downloads/Packages										Change
	Ask where to save each file before downloading						ON

Accessibility
	Captions
	Add accessibility features
	Open Opera Addons page

System
	Use hardware acceleration when available							ON
	Use DNS-over-HTTPS instead of the system’s DNS settings				OFF
	This functionality uses third party services. Please read our Terms of Use and Privacy Policy to learn more.
	Open your computer's proxy settings

Shortcuts
	Enable mouse gestures												OFF
	Enable rocker gestures												OFF
	Hold Ctrl+Q (or press it twice) to quit Opera						OFF
	Enable advanced keyboard shortcuts									OFF
	Configure shortcuts

Reset settings
	Restore settings to their original defaults

In spite of this, whenever I reopen Opera it does not fill the screen vertically. The tabs I have open when exiting and restarting are:

• Duck-Duck Go
• GitHub
• Opera Forums
• Settings

There is no tab that shows speed dial.

@sgunhouse
Okay. There doesn't seem to be a way to disable speed dial other than to hide it; will that be sufficient to my purposes?
How do I 'make a session'? There's nothing in Help about that.

Addendum:
After closing and reopening Opera, with no tabs showing the speed dial, the page still opens with only part of the vertical space filled. How can I get rid of speed dial if that's what's preventing Opera from remembering the window size?

@sgunhouse
Hmmm... I don't see "Continue from last time" anywhere in settings. In "On Startup" I have selected "Retain tabs from previous session"; is that the same thing?
I don't find anything at all for "saved session" in Help.

@leocg
Okay, I'll look into that.

I have a strong aversion to Sans-Serif fonts, so I want to change all (but Monospace) fonts to Serif variants (specifically, Droid Serif. I have set the values in Settings -> Basic -> Customize Fonts, but after reloading Opera the web pages still are rendered in Sans-Serif.
I'm migrating from Firefox, where the fonts setting has a checkbox, "Allow pages to choose their own fonts, instead of your selections above", which I have always disabled; but Opera doesn't seem to have such an option.
How can I override to Serif the font family settings on the pages I browse?

Whenever I open the Opera browser on my desktop its window is only half the height of the screen. With other applications I can use window manager controls to force the window to full height, but Opera does not honour the window manager's hints.
How can I tell Opera to maximize vertically?
(N.B. Don't tell me to use full-screen mode; that would be a waste of screen real estate on my ultra-wide hi-def monitor.)