Buying rear rack during pandemic crisis - go real bottom, up high, or wait?
25kg on a rear rack is already very hard to handle, especially if it’s on the top of the rack. Start leaning the bike unconsciously while standing at a red traffic light and you’ll suddenly do a very embarrassing dance trying to prevent the whole thing (+you) from toppling over. Carrying more than 25kg on a rear rack is really not a good idea. I doubt the bolts and bolt holes of the frame can survive much more. It’s also quite hard on the rear wheel, so make sure you have a sturdy rear wheel if you plan to carry this much load.
I never had a use for the spring clamp on my first rack and removed it after some time for weight savings. It’s really only useful if you quickly want to transport something small and easy to clamp, like a sweater. Even then it’s hard to keep it away from the rear wheel. I’m commuting and traveling by bike and I’m more than happy with my Ortlieb Backroller panniers (Vaude makes very similar ones). They are extremely sturdy, roomy, waterproof and quick to take on and off.
As Qwerky mentions in the comments, bungee cords are much more useful and flexible for securing things to the top of the rack. I’m actually using the detachable shoulder straps of the Ortlieb panniers for the purpose.
One more tip from experience: Tighten the rack bolts to appropriate torque and use threadlocker, they tend to come loose.