Are Shimano R7000 chainrings (50 and 34T) compatible with Ultegra R8000 crank?
Yes, they are 100% compatible. See https://si.shimano.com/api/publish/storage/pdf/en/ev/FC-R7000/EV-FC-R7000-4331A.pdf https://www.bike24.com/p2284436.html
R7000 &amp; R8000 chainrings are a 'B' match, that is different materials but compatible.
With a 50/34 compact, then the rings are 'MS' pattern. Previous generation are 'MA', which is incompatible.
There is a high chance you can use the even cheaper/heavier RS510 chainrings, which are also 'MS', however I haven't seen that confirmed.