Have been through SFO more than a few times on United and partners, I think I've been to all the United lounges there.
I don't suppose you have a business class ticket on this trip? If yes, then you get to enjoy the United "Polaris" (business class) lounge there, which is surprisingly awesome (free food, which is quite good, in their sit-down restaurant section, showers, very comfy seating area, bar, and more). If you have a business class ticket, the SFO Polaris lounge is a wonderful place to kill time (only available if you have an outbound business/first class ticket from SFO on United or one of their partners, and they are strict about entry requirements, so there's no chance of trying to sneak in if you don't meet their criteria).
If you're not on a business class ticket, then it'll be the "United Club" lounge for you. It's perfectly adequate, though a far cry from the Polaris lounge. The United Club has some food (typically a limited menu of serve-yourself options, which are reduced/simplified during the pandemic), fairly decent stuff but nothing to gush over. There's also a bar, multiple seating areas, it's not a bad option. 7 hours would be a long time there, but it beats sitting at a gate for that long. I would probably take breaks, wandering around the airport, shopping and people watching (at least I would if a pandemic wasn't raging). There should be no time limits that I'm aware of, you have passes, they should be good for the whole day, and killing time there between your United flights is the reason it exists.
Not sure about having to re-clear security, you might. You will be coming in on a domestic flight, departing on an international one, and I believe United's domestic and international gates are in different wings of the terminal, so don't be shocked if you need to go through security again. You certainly have enough time to do that.
One last suggestion: since your flights changed (and significantly), you do NOT have to simply accept the options United has offered you (it was picked by a computer). If you haven't yet, give United a call. Explain the situation, ask (nicely) if they can accommodate you with a better option (be prepared to suggest what a "better option" means to you - eg shorter connection, maybe leaving a day earlier or later, flying to Frankfurt on a partner -- Lufthansa flies from SFO to FRA but check details like seat dimensions before committing). Though you booked with miles, you're still a paying customer, and they can (and often do) bend the rules somewhat, to get you a better flight. Sometimes, after a schedule change you can end up with a much better option than you originally had, so it's always worth trying.
Hope that helps. Good luck.