Get.Swifty, I live in a hilly area, so this may be relative. We have hills up to 20%. Most are in the 5-10% range.

My geared bikes are 55/39 and 9-23.

You are correct – it looks mostly like rollers with an occasional climb.

My setup for this ride is 44×21 single speed. Momentum will be important for me. I only expect to grind on a few hills.
49×26 should be ample for the biggest climbs.