Firefox has my very favourite vertical tab system of any browser in the Tree Style Tab addon: https://addons.mozilla.org/en-US/firefox/addon/tree-style-tab/
I’m not too sure how to simplify jumping between profiles though. I haven’t used it so I can’t vouch for it, but maybe the Profile Switcher addon would work for you? https://addons.mozilla.org/en-US/firefox/addon/profile-switcher/
Not sure how they’re planning to compete with the price of the lower end Steam Decks with those specs…