The standard Bosch type relay uses a 1/4" qd fitting aka "faston" fitting. I don't think you'll find those in 8g crimps.
Don't confuse the carrying capacity of the wire or relay, with the need to use heavier cable to prevent voltage losses. If a 30A relay is sufficient for the job, and you are only using 8G cable to prevent voltage losses in the cable run, then there is no reason to worry about putting a smaller fitting on the end of the 8G wire so it can plug
into the relay. You can indeed trim back, unstrand, some of the 8G cable until it fits into a 10-12G "yellow" faston crimp. That may not be ideal because the heavy cable may tend to pull out the connection from the relay. A better way is to buy a relay SOCKET, which has wires pigtailed into it, and then connect the 8G cable to the pigtail.
Last time I checked, all of these relays had cheaped out and they now are usually constructed by passing the contact through the casing and forming part of the relay with the same piece of metal. As the relay ages and the contacts wiggle loose in the plastic, they can cause the contact to open internally
, inside the relay case, when the heavy wire pulls on the external contact. So a relay socket is highly advised unless you really ensure the heavy cable leads are stress relieved and secured.