In my user case I haven't found any alternative that really fits my needs.

In software applications in general, I never find one that's better than all others in all respects. It always comes down to what is best overall. And that changes over time. Right now Vivaldi is my default and Opera is No. 2 (or sometimes Brave if I want to quickly check something when I'm unauthenticated).

But it used to be Opera 1, Vivaldi 2. Before that it was Firefox 1.

This is on desktop. On mobile usability and feature differences aren't as big an issue.

For me, Opera is the best blend of simplicity and features, though.
I had used Vivaldi such a long time, but it has so many features, I don‘t need. And the customization thing drives me nervous.
I used Brave for a short time, and it lacks so many features that I take for granted.
Last was good old Firefox, but it was lagging, especially when filling in forms. And it made my fans roar. I really wanted to like it. But it didn’t deliver, at least not for me.
Edge was fully bloated with Microsoft stuff, that it‘s hard to bare, if you don’t like Redmont that much.
I‘m even 56 by the way.

• I didn't know until last week that people behind Vivaldi worked at Opera in the past, so I hoped for similar features and was not disappointed
• I for one don't care it is not 100% open source
• It is actually modular like the original Opera used to be (as in, you can place tab bar, address bar, etc. wherever you want, add or remove buttons to them wherever you want. It's basically the same UI building experience from the original Opera)
• It has a more sober UI
• It has a more customizable Speed Dial (as in you can change tile size and have folders inside folders)
• It can also pop out videos
• It can also group tabs (and it provides multiple options of how it's displayed)
• I didn't test the sidebar yet, but it not being the way I want isn't a deal breaker anymore. I've been living without a sidebar for a while now in Opera because it can't deliver what I want, which is the ability to add unlimited pages to it like Edge and Yandex do
• It is possible to set pages to open as an app and pin them to the taskbar. This is great for web apps like Outlook, Spotify, WhatsApp, Google Keep, etc. I've been using Edge for this feature for a while and now I'll finally be able to use only one browser for all my daily routine (I can't use Chrome and Firefox for my routine because I'm a programmer and use them to test my software. So browsing data and extensions are fully zeroed all the time in those two)

Just wondered if someone found some alternatives to Opera One in which there's no bubbly UI, no extreme acid colors, no splash screens and sound on updates, no redundancies in borders, workable edges for scrolling etc.

Hmmmm. I don't see all that splashy-glam that you mention. Maybe it's because I run it on openSuse Tumbleweed.
I notice most of the "splashy" complaints come from folks running the WIn OS.

Granted, Opera is not all I use .... I also use Chrome, FF, Brave - each has their own special purpose.

Best of luck in your search!

Brave is a good option, but it's worth noting that it's still built on Chromium, and it includes cryptocurrency features.

So, Brave is indeed a very good option Always full of surprises:
https://www.theverge.com/2023/10/20/23925192/brave-browser-vpn-windows-11

If you're looking for something like Brave but without the cryptocurrency aspects, you might consider Tempest. However, it's important to know that Tempest isn't open-source like Brave.

And if you prefer a browser that's not Chromium-based at all, you could give Floorp a try.

@saeedp There's also Firefox and DuckDuckGo, if you're interested.

Firefox is still a very good option. DuckDuckGo as well, but especially on desktop it‘s very very basic, unfortunately.
The best alternative to Opera seems to be Vivaldi, but of course it could be way too much in festures and customization.
Edge would also be a good choice, if it wasn‘t Microsoft being so absolutely intrusive.
So, if Opera is being stabilized with version 103 or so, it‘s promising to be the best middle ground of all these choices. At least, I hope so.

And what kind of idiot thought it a good idea for a browser to make noise when you start it?

Is the "reinvention" ideological coup among Opera devs?

If I were to use an alternative to Opera, I‘d be giving Vivaldi a try. They haven‘t been involved in shady practices like Brave has. I wish, the browser wouldn‘t be that buggy.

I don‘t think that Opera One isn‘t that bad, actually. But when it all boils down, you are probably left with Edge, Chrome or Safari, maybe Firefox if you don‘t want to use Opera but also not some sectarian and outlandish offering like these one man or woman projects.

Brave (with all the crypto junk turned off) + this addon on for a VPN proxy similar to Opera.
https://chrome.google.com/webstore/detail/touch-vpn-secure-and-unli/bihmplhobchoageeokmgbdihknkjbknd

No workspaces as such, but 'tab groups' works as it should (unlike 'tab islands').

I switched to that as my primary business browser after the "reinvention", but after the loud splash screen annoyance dropped, it's my primary for recreational use as well.

Posted from Brave, BTW.

If you like the AI capability, Slimjet has it.