I wouldn't even replace it with another RE050 unless the tread wear is almost nothing; 7K miles is not nothing
Get two new rear tires. I would suggest getting four new tires if you don't like the RE050s, but of course, the issue is cost. Perhaps you could do that and then recoup some money for the three lightly used RE050s.
While I cannot recommend it due to potential hazards, I have run mismatched tires on a couple of occasions on the Z without incident. But the wrong combination of tread patterns between the two different tires may cause serious problems in wet weather.