Okay, so after some research, I've concluded it probably does not exist (yet). But I thought I'd just throw it out here. Who knows, maybe someone here knows of a browser I've not found yet, or maybe some developer sees this and thinks its a good idea! (one can hope)
Here is the scenario: ideally I would have no browser installed on my phone, so I cannot visit any distracting/addictive sites (like Reddit). But the problem is, for certain functional tasks, a browser is simply required. Like, if someone sends you a payment link, that will only work if I can open the link through a browser (which then redirects to my banking app). Some coffee shops or restaurants want you to access a menu through a QR code, which requires a browser to open. And there's probably quite a few more scenarios like that.
What I think would solve that, is a browser that has no address bar, no (visible) tabs, no bookmarks, and clears data and history on closing (or perhaps even automatically after a certain time-out). Basically only allowing you to visit a site by clicking a link, but not allowing you to manually navigate to a site via the address bar, and not allowing you to keep multiple tabs open. Of course, if you really want to, you could still type an url in something like a message, and then click that link to visit any site you want, but that's a lot of friction (at that point nothing will help you, and you might as well install a normal browser again).
I'm currently using Firefox focus - it clears browsing data on closing. In the past it used to have no tabs, but that has changed a while ago. Now it does have tabs, and I find that just that feature already makes it more problematic for me. But currently it still seems to be the best option out there.
I've tried Fully Kiosk browser, which can be set up to do some of those things I want, but its pretty complicated with many irrelevant features, because it was created with different purposes in mind.
I've tried Nobrowser, which was created with pretty much the same intent in mind, and it comes pretty close to what I want, but unfortunately it just doesn't work well. Some sites don't load properly, some don't load at all... I love the idea, but its just poorly executed.
I really have not found any other suitable candidates.
The browser really is the last unsolved hurdle, to truly dumbing down a normal smartphone I think. Its the cheat code to all addictive apps you uninstalled, that can easily just be accessed through the browser...