A 235/35R18 would be the closest in terms of diameter, however isn't a super common size compared to the 225/40 (limiting tire choice). Being that your next set will be 8" wide; the 235 may be a little on the wide side.
A 225/40R18 is the correct width for an 8" wheel, and the added sidewall height will only help counter the increased harshness by going up in wheel diameter.
We run 245/40R17's on our shop BRZ, which is actually even larger in diameter.
Edit: 245/40R17 and 215/45R17 are within a few millimeters in diameter.