I don't know what else to tell you, you're not even paying for the browser, it's a free service. Any browser that implements an ad blocker is probably also going to have a lot of features you don't use

so use the --disable-features flag at runtime?

maybe go into brave://flags and disable the wallet?

or just admit you're only against Brave because Brendan Eich privately donated $2,000 to a failed CA Proposition?

Right now, I have ~110 tabs open, and the "Extensions" item in about:processes shows ~400MB used, but that's all extensions, ~10 in total, not just Sideberry. But to be honest, I'm not sure I'd notice performance issues until too late, as I'm sitting on quite a bit of RAM already.

Easy, change the userChrome.css to this:

    $ cat ~/.mozilla/firefox/$profile.default/chrome/userChrome.css

    #main-window[tabsintitlebar="true"]:not([extradragspace="true"]) #TabsToolbar > .toolbar-items {
      opacity: 0;
      pointer-events: none;
    }
    #main-window:not([tabsintitlebar="true"]) #TabsToolbar {
        visibility: collapse !important;
    }

I hate over-regulation, especially made by people that have no clue about the market they are regulating, but as you said, I think in this case it's more than warranted.

Eye-opening and pretty straightforward with e.g. mitmproxy. Strip TLS with a temporarily trusted self-signed cert to get the whole picture.