Julian Radford recommends the following way of doing the Epte Valley and Vexin without too many hills:

We left your route just after St Germer de Fly on the D102. There's a long and glorious downhill from Puiseux en Bray to the D915. We made for the Voie Verte via Serifontaine and Gisors using mainly quiet roads apart from a mile or so of the busy D915.

We took the Voie Verte as far as Bray et Lu, then heading off again via quiet roads to Meulan-en-Yvelines via Chaussy, Aincourt and Oinville-sur-Montcient. I'd really recommend this route, its quiet, beautiful and there aren't too many hills, although its a little further than your route I think. For those interested in Urbex, there's an abandoned sanatorium at Aincourt.

We rejoined your route at Triel sur Seine