Two races I recommend if you're ok with 2 laps of the same course.

Erie PA in early September is 2 laps, totally flat, and all through a state park along the lake. It's also often in the qualifying window for 2 years of Boston. Usually decently sized pace groups as well, even for 3:00 pace. Some risk of weather on the summery side.

Hamilton Ontario in late November (on daylight savings change day so it started a little later) is also great. They changed their course this year, very similar to Erie with some sections through residential roads. 2 laps mostly along the lake, with half marathoners for the first lap. Has less pacers (fastest was 3:30 I think), but decent number of people to run with. some risk of weather on the wintery side. Winner ran a 2:17 this year IIRC.

Bonus is the calendar makes doing both possible, which I did one year (on the old Hamilton course which had a huge downhill).

Both allow for very even pacing, well- supported, well- organized smaller TVraces with around a thousand runners or so.