I think Amelia and Grace are quite classic names that happen to be very popular - so more like the Laura and Sarah of the 80's, than the Gemma and Nicola for example. I suspect they'll still be used in reasonable numbers 30 years from now , even if/when they fall out of the top 100 so I wouldn't panic too much about them dating imo.
But OP if you want a first name less overused currently- instead of Amelia consider Aurelia, Amara, Julia, Lydia, Victoria, Celia, Cecilia, Cordelia, Lilia, Claudia etc. Instead of Grace - Greta, Rose, Maeve, Eve, May, Faye, Hope, Faith, Verity, Felicity etc (some of these are top 100 but no where near as popular as Amelia and Grace).