I have owned, or own Grado SR60/80/125/225/RS2/RS1, the Fidelio X2HR, the AKG K 701/K702/Q701/K712, and am familiar with the Beyerdynamic DT770s. I don’t know much about the Sivga SV021.
The Grados are FUN. A really enjoyable listen, easy to drive. Some may find them uncomfortable, and the build is pretty retro. I’d say go right to the 125, or used 225, RS2 or RS1 if you can swing it.
The AKGs are not a difficult impedance but this is deceiving- they don’t really perform unless you’re using a healthy headphone amplifier. This is required to get them to show their capabilities. That said, if you drive them properly, you’ll be in for a treat- a pretty neutral sound and n exceptional soundstage. Very comfortable. Don’t rule out the K702 and K712, or used Q701s.
The Fidelio X2HR- excellent bass performance, especially considering they are an open design. Easy to drive, like the Grados. Good detail. All around solid performer. A bit heavy to wear for long periods in my experience, and sometimes the build quality/quality control can vary, and despite the solid feeling build, I do worry about their ability to take any wear/tear/abuse. Ear pads are lint magnets.
The Beyerdynamics- these are closed or semi-closed back, so if you’re trying to listen to music with folk around, these will isolate you from outside music, isolate your music from others, the best of your choices here. All the others are considered open backed and let sound leak in and out- but open backed cans are, well, more “open” sounding, offering on average a better soundstage and listening experience. Closed back will however be the choice for noisy environments.
What did I end up keeping myself? I do have some vintage Sextett AKG K240s (very demanding to drive), but for ones you listed, I kept the Grado RS1i or e (can’t recall), and the AKG Q701 and K712. Nothing within the price range of these has been able to replace them- I adore them.