Axios I have a feeling you and I visit some pretty unusual privacy-oriented sites and test sites with a ton of info. Firefox (I left anrkenfox in) and Brave are two great options. But Libre Wold is in my rotation. I trust it. Most of the places I go read about trust it.
I will continue to trust it. Their simple goal is to be the most secure browser while walking that razor's edge by making you not stick out. It's a weird edge to walk and I wonder if it is doable. But again: I feel like I can do so much more with the Firefox about:config
than Libre does.
Mullvad gets a LOT of love but I cannot press it into fulltime duty. It's made to be a Tor browser without the Tor and made to be used with (their) VPN. I like it, it's smartly locked down...but I can't use it daily. I don't know why.
Ungoogled-Chromium
I use daily and trust fully. No one throws shade at it....but I have noticed it does not pass the tests very well...but I read a lot of wireshark-type tests and Ungoogled is one of a handful that make no outbound connections on boot/no phoning home/etc.
You say "it's all relative" and I agree: they were all built to do different things very well. What the hardcores (I ain't there yet) always refer to as your "threat model" even though I'm not sure I have one. Just trying to keep the gigantic digital dossier they keep on everybody as small as possible is my only motivation🙂.