I’d personally keep the 650, If you only have 28k miles on it it’s still like new ! Treat yourself to a new exhaust or something if you want to bling it up. Spend a few hundred rather than £2.5k and you’ll have another year at least of fun
We ride two up on the 650 all the time and have no problems with lack of power. I’ve dropped the bike a few times on rough trails, it’s heavy to lift, even with two of us. I imagine a 1000 would be far heavier. Suppose it depends on the type of riding you do really