Normally this is due to a shorted load and since the only true load on this circuit is the starter, I would start with replacing the starter if it were me.
However, if you want to prove this out then you would need to test the circuit for a short to power or ground. Also, you could install a smaller fuse inline between the fuse and the switch, and the relay and the starter. I marked each location on the last attachment.
Then when you turn the ignition on the issue is on the side that has the lower amp fuse that blows. So if the starter is the issue then the fuse in number 1 will blow.
Let me know if you have questions on this and we can go from there.
Images (Click to enlarge)
Saturday, March 27th, 2021 AT 3:42 PM