I mean, it is called /r/all for a reason, though I'm not sure what algorithm is used to determine what actually shows up. I try to avoid /r/all, since it seems to basically be a hub of shitposting, low-effort content, and random vitriol. If you do want to use it, you can try blocking particular subs using RES, making your own multireddit, or following a multireddit made by someone else.
Those multi things sound really neat. I forgot about them entirely. Thanks for reminding me of that! I'll probably switch to it once I accumulate enough good subreddits in the mix. I heard a few weeks ago that RES had some issues with the Reddit admins - or, rather, vice versa. Is it still good enough to use and not worry about stuff popping up unexpectedly?
I haven't had any issues with RES, and have been using it consistently for years. Possible that I've missed something, though. If you add together all of the default subs and a bunch of news subs, you'll probably get something pretty close to /r/all. I think multireddits cap out at 50, though, so you might have to be a little bit selective.