If you already have an endurance frame that fits and is in good condition, with what I assume to be entry level parts, and want something that rides better on your longer rides, then you could also consider spending that same budget on upgrading the groupset, lighter and lower pressure tyre & wheel combo with wider internal width, premium carbon shell or 3D printed saddle for buzz damping/comfort, and perhaps even new bars and stem on your existing frame. That way you can source bits that suit your riding perfectly and very likely shave considerable weight to change the responsiveness of the bike.
My impression is that said bikes in that price range leave a lot to be desired in some of these component categories, which are ultimately critical to the way the bike feels and performs. I would give the components a go first and then if you still lust after a new frame for some reason, just buy the frame and pop your bits on it. It's not hard to beat the stock components at that price point if you are open to a bit of research and sourcing.